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View Full Version : Boost leaks, what they do and how to deal with it



Vernon
03-05-2003, 04:03 AM
Checking for and removing boost leaks using compressed air.

The following is from a post I saved describing leaks to someone who found his Supercoupe produced more power on lower octane fuel. This was obviously confusing. There is a usual cause for this with forced induction, boost leakage. This is very common on our cars, especially at their age and mileage levels nowadays but usually goes unoticed. Here's the problem and reason it could use 89-octane fuel and actually run better with it than 93. All the air that the supercharger is pulling through the mass air meter is measured whether it gets combusted by the engine or escapes back into the atmosphere through a leak and therefore the EEC computer is adding excessive fuel to the engine because it doesn't account for any boost leaks. The oxygen sensors are ignored under wide-open throttle and the air fuel ratio is calculated for a rich mixture based mostly on the mass air readings. So what you end up with is an extremely rich full throttle mixture that not only cools the cylinder more than necessary but also generates much lower than optimum power levels and less combustion heat than normal ones. The largest factor in engine octane need is air charge heat inside the combustion chamber. Octane is not fuels power rating, rather itís a volatility rating, or it is a measure of the fuels resistance to combusting. Actually lower octane fuel burns quicker and hotter. The higher the octane number the more stable fuel is and therefore more resistant to self-ignition. In this theory of a super rich air/fuel charge, a lower octane fuel burns faster and hotter, therefore perceptibly producing more power than fuel with higher octane. In an engine that doesnít need higher-octane fuel you normally wouldnít be able to tell the difference between the octane ratings. But with a properly tuned high-octane need engine lower octane fuel usually doesn't work.

Most of the time boost pressure leaks from the ends of old vacuum hoses that have gotten soft and swollen. Check the 3/8Ē hose connecting the intake plenum in the back, if it's easily pulled off by hand it positively leaks at 15 PSI. Under vacuum leaking fittings can pull together and seal up so it runs fine. Donít be pacified by normal boost readings, even with some leakage the supercharger can maintain 15 PSI because the engine and some small leaks are still within the superchargers flow capabilities. Look at your tailpipes and the area around their exits; is there a heavy soot coating? My advice to you if you see excessive black soot in the rearview mirror at WOT (especially at night with someoneís headlights right behind you), soot build up around the tailpipe exits, and loose fitting vacuum lines is to take an hour to search for and eliminate any possible leaks. Give this a try as not only have I done this on quite a few Supercoupes with very impressive results, but others I have shared this method with have reported back giving thanks and having a blast with the new found power and fuel efficiency for less than $15 and an hours work.

Just so everyone understands, when you do this the entire intake will be pressurized, from the back off the throttle-body to all cylinders with an open intake valve. The idle air bypass valve will be closed with the key off so it is of no consequence for this test procedure. The IAC solenoid is spring-loaded and shuts without current. This makes the system somewhat airtight and allows pressure to build without excessive air escaping. Both sides of the bypass valve will see equal pressure so donít tie it open. Use an inline pressure regulator like those used for air gun painting with an air compressor. Set it on 20-PSI maximum as excessive pressure could blow out seals and gaskets so act in a prudently careful manner. One regulator would work but two in series are better for pressure stability and safety. Run an air hose to the regulator hung under the hood with a wire hook. Find a hose barb fitting and a reducer fitting to thread into the regulator outlet port and clamp a section of small clear vinyl tubing onto it. Run the tubing into the intake manifold at the boost gauge nipple, as this seems the most unlikely place for a leak to develop or go unnoticed. Once the airline valve is opened the intake will pressurize to 20 PSI. A slight hissing can be heard at the throttle-body as pressurized air leaks through it. Using a plastic spray bottle filled with soapy water saturate all possible leak points. It helps to use thick film soap like laundry detergent. This is easier on a cool engine. On mine the large hose nipples at the back of the intake and the PCV line joints in the back were leaking a shocking amount. Another car Iíve sealed had a large one at the PCV valve opening itself. Be sure to check every possible point such as: fuel injector o-rings, intake gaskets, all tubing joints including the spanner nut, the vacuum tree on the firewall, etc.

If you find similar leaks and chances are you will, first replace the large engine mounted vacuum hoses and use screw type hose clamps at all large fittings to seal them permanently. Get a handful of small zip-ties to use on all the small vacuum line fitting that hold boost pressure. Installing a new Motorcraft Supercoupe specific PCV valve should be considered mandatory. It is designed to seal boost and many aftermarket PCV valves arenít capable of this. The correct valves are rarely in stock under any brand; beware if they donít have to order this part. Also make sure the inlet tube clamps are very tight to avoid vacuum leaks between the throttle body and the MAF.

At this point seriously consider changing the spark plugs, as they will already be fouled out from the rich mixtures preventing full performance. Since Supercoupe engines have a low compression ratio Ford specified emissions type spark plugs that are notorious for misfiring under boost. Their extended tip design is to place the spark kernal in the center of the charge with dished pistons but it under power it absorbs too much heat being exposed like so and promotes detonation. The extra wide gap helps to positively ignite leaner air/fuel ratios under light loads thereby preventing misfires at those loads but it produces the opposite effect under power when cylinder pressure is high. I recommend using Motorcraft AWSF-32PP plugs, these are the standard 3.8 double platinum plugs. Their smaller tip design remains flush with the chamber roof thereby remaining cooler and reducing pre-ignition. Also they have a smaller gap, about 0.047Ē, which greatly reduces misfiring under boost. Both are good things for any supercharged engine, proven to improve efficiency and power under boost without a significant emissions penalty. For a manufacturer who is producing thousands of cars it mattered but for your emissions testing these plugs wonít cause a failure unless your car was already on the edge of passing, in that case you have bigger concerns than normal type plugs. The most important reason to prevent misfires under boost is so you will be able to accurately judge air/fuel ratios. Rich ratios on the dyno or excessive soot in the exhaust gasses are more often from misfiring plugs than boost leaks, but most cars have both until corrected.


After sealing a leaking intake system like this you will notice an immediate difference. The increase in part throttle torque is phenomenal along with better mileage and higher power levels across the entire engine range. This higher power output is due to a more realistic air/fuel ratio and should generate the need for higher octane again with boost, but the increase in mileage and power will even out the cost. You will surely be surprised and pleased with the new character your engine develops. In my experience part throttle torque and throttle response improved about 20%. The engine felt like it got another cylinder. The throttle became very responsive and more precise. The engine runs much smoother and gets better mileage. Less carbon in the WOT exhaust. Boost, while unchanged at peak, peaked sooner and held longer. The WOT power improved as well and the car sped up but not as much as the part throttle improvement, more like 10%. With a 5-speed the car will be easier to drive away from dead stop.

The reason being is two-fold. Not only do boost leaks rob power producing pressure but all air going through the mass air is metered for fuel requirement whether it gets used as charge or blows out as leaks. Therefore, with leaks the engine always runs rich causing lower power and crappy mileage when using some boost. Because the blower can outflow the leaks and engine, boost will still peg at expected levels but the blower will be pumping a lot of air through the mass air that doesn't stay in the intake. The fix traps all air and restores factory fresh fuel ratios.

Vernon C.

Thomas A
03-06-2003, 02:43 AM
TTT

This is good info, maybe someone can add it to the FAQ's section.

Thomas

buck92sc
03-06-2003, 03:20 AM
That could be the answer to all my problems! I cant wait to replace all the vac hoses and get a ford PCV valve, i can hear mine hiss, its aftermarket. Thanks great info Vernon, should def be a faq.

Kyle

Bill McNeil
03-06-2003, 03:52 AM
Great info Vernon. Nice contribution to the community. I'm going to save this for future reference, to give to others if the question arises. You should hit the TBSCEC with this also, Vernon, so that add is can be added to the Misc archive.

I think Ron should keep this for a technical archive here also.

tbirdsc35th
03-06-2003, 04:01 AM
Vernon, that's an excellent article. Being way more supercharger literate than I am, I had not thought about the problem of boost leakage nor the situation of the SC specific PCV. I know it's always a good idea to have good vacuum hoses and connections. This summer I want to remove the "extraneous appliances" from my engine and clean and detail it. I will look for these areas you describe in addition to the general vacuum connections. Well done.

I hope you have a great week.

vancouverBC
03-06-2003, 04:15 AM
Thank you, for the insight that you've put forward, it makes a lot of sense. I plan to try it as soon as I can. But I think I'll seal off the t-body first. Up to now, I've only used propane & intake cleaner to look for leaks and maybe this method will reveal areas that could have been missed since the engine doesn't need to be running for this search. Hopefull, it will allow my stock 90SC AOD to run as well as it could.

Superloop
03-06-2003, 03:16 PM
But, what regulator do I connect the compressed air to?
Sorry Im a little insane.

1BADSC
03-06-2003, 05:06 PM
great article. TTT

Vernon
03-06-2003, 08:34 PM
Thanks for all compliments, I hope it helps many of you.

I did write it for the TBSCEC FAQ because a few of our colleges remembered it and were referencing it to another poster. I figuired I should clean it up and repost it again.

Superloop, use an inline pressure regulator like those used for air gun painting with an air compressor. This is an compressed air accesory, not part of the car. You can get them at stores that handle air compressors and tools. Make sure to get one with a gauge or add one to it.

If a moderater here wants to make a FAQ out of it they have my consent. I think everything you post becomes the site's property anyways, but to be nice.

Vernon

gldiii
03-06-2003, 08:53 PM
Thanks, Vernon. The post has been copied to the FAQ section of this site.

Superloop
03-07-2003, 04:16 PM
Ok. Thanks for clearing that up.
I got another question, where is the NIPPLE?.:p
I have trouble finding it
Thanks in advance!

Vernon
03-07-2003, 08:46 PM
On the top of the motor in the rear, towards the drivers side. It comes out of the intake elbow and points towards the brake unit. This is a metal nipple about 3/4"to 1" long and 1/8" diameter. It has a small plastic line with a rubber end attached to it, this is the line going to the boost guage. Pull the line off at the rubber end while performing this test and reattach it afterwards with a small zip-tie pulled onto it to help seal it.

Vernon

Vernon
03-09-2003, 09:49 PM
"Thanks Vernon for your invaluable information in the
FAQ concerning vacuum leaks. If only the Ford
mechanics were half as intelligent and thourough and
half as cocky.

Anyway, just a quick question. I have a 94 SC auto
with a very hard-to-find leak (I will try your methods
soon). Will I get a EEC code if what you explain as a
rich condition during vacuum leak situations is true
(from the O2 sensors)? I have a new downstream sensor
on a rebuilt motor, and the only code I get is
"excessive EGR flow" but only on really cold mornings,
snow and ice. Thank you!

--Alan"


Alan thanks for the words of appreciation.

Concerning your question I don't think the situation I went over in the FAQ is what you're experiancing, though the testing method will help you find vacuum leaks also. A lot of times there may be a leak with pressure that holds vacuum in the same locaton, a boost leak. Vacuum leaks will go both ways, under pressure (boost) and under negative pressure (vacuum). That is why I shared the technique with everyone, because without using pressure many boost leaks will go unfound.

A boost leak will never set a code because the EEC always expects a rich full throttle mixture, also normal O2 sensors like those used in the SC are very presice when giving degrees of leaness because they generate voltage when exposed to the unburnt Oxegen, the more O2 the higher the voltage. But once the mixture goes rich very little O2 is left in the exhaust if the burn is sucessful, since there will be very little O2 to measure in a rich mixture the O2 generates very little signal. What this means is the EEC knows it is rich but it really doesn't know how rich. So as long as it can maintain a even mixture rate at idle and low speeds it doesn't worry about how rich it is when it's supposed to be rich.

Now a vacuum leak is a situation when the pressure inside the intake is lower than the outside air. This occurs when there is a breach in the intake and the engine is at idle or light throttle loads (under vacuum). Outside air pushes into the system without getting read by the MAF sensor. This will generate a lean condition as the engine is burning more air than what the MAF sensor is reading. The EEC programing adds the proper amount of fuel for the MAF reading but there is still too much O2 left in the exhaust as read by the O2 sensors. When this excess O2 exceeds a set limit for enough time a "(R or L) bank O2 too lean" code, sometimes both R and L. This would reveal a large leak or a fuel delivery restriction of some type.

The code you have would indicate a EGR valve that is no longer sealing off or a deffective EGR position sensor. Check the position sensor first. When measured with a Digital Volt and Ohm Meter DVOM set on low volts 0-10 or 20 it should read 5 volts at all times on the VREF or input line, the output or generated signal line should read 0.2 Volts with the valve fully shut and 4.9 V with the valve fully open. Measure this while the engine is cool enough to touch and the key in the on position. If these readings are in specs replace the valve. A leaking EGR would act like a vacuum leak but without adding excess O2 into the intake as the gasses leaking into the intake have already been burned once, therefore it would leak but not set a lean condition. Under power exhaust pressure will be higher than boost in most cases so it would still be a negative type leak, though technically not because of vacuum.

Vernon

silversc90
04-09-2003, 10:34 PM
vernon, both my 94 five speed and my son's 90 auto do the smoke out the tail pipe thing. i have only owned my car since october of last year, so i know very little about it. on my son's car we know a little more. head gaskets,tranny,and some other stuff. to make long story short, we changed plugs, if anything they show a lean condition, light grey color. your opinion would the plugs clean themselves in a part throttle load? thank you for the very informative post, our cars will be checked out and repaired if needed. thanks again.

Vernon
04-10-2003, 07:05 PM
Silver, this over-rich condition will only occur under boost and usually the more boost the worse it will be. Stock gapped plugs only worsen the soot at WOT due to mis-fires. But at part throttle the O2 sensors are keeping a stable slightly lean fuel ratio as long as leaks are within adaptive limits, and most are. Unless you took your car for a WOT throttle blast about a quarter mile long then shut if off while still at WOT, pulled over and inspected your plugs you won't see much carbon on the plugs. (Don't do this with aluminum heads by the way) Plugs do clean up at normal throttle positions. So quickly in fact that if you did this at the track and drove into your pit and pulled a plug it wouldn't be an accurate example after the short drive back. Just remember if you can see it (soot), it's there.

You will be happy with the results based upon that alone.

Good luck,
Vernon

silversc90
04-10-2003, 08:19 PM
thanks vernon,

tbirdboy
04-10-2003, 09:45 PM
Hello Vernon,

Tried your procedure as described as I noticed a small leak at the spanner nut using soapy water. I do think I should've used 2 reg's instead of 1. Seems any fluctuation in the compressed air side would cause the needle to flicker past the 20lb mark which I had been cautioned to avoid. And I will remove the plastic cowl molding next time to get a better glimpse of the entire vaccuum line assy. Thanks.

silversc90
04-13-2003, 10:00 PM
Found 2 leaks, short rubber hose between the bypass, and the butterfly shaft. Everything else was sealed up fine.

DrFishbone
04-15-2003, 11:03 AM
what kind of damage could you expect if the pressure went above 20lbs? like, what sensors, valves or gaskets might be damaged? i have a feeling my regulator was junk :( thanks!

derrktor feesh

Vernon
04-15-2003, 08:55 PM
None really, but there's always a first time. I just wanted to make sure somebody doesn't run full pressure and blow injector orings, intake gaskets, etc. I did this today and because my regulater is single now and sticky I spiked 30 PSI. No worrys. At twenty there were no leaks of significance so unless you find a big leak that couldn't have been there before you didn't hurt anything. Remember some is always leaking past rings, the throttle etc so a surge big enough to do real damage would be hard to come by.

I must be getting better since I had no leaks on my latest install. :D

Vernon

DrFishbone
04-15-2003, 09:52 PM
i wondered if i might've messed something up after letting about 30 psi through (i'm estimating) i didn't think it would hurt, but was jsut looking for a second opinion :) something (probably a sensor) is a little screwy after i put a newer sc (with intake plenum and tb) in this weekend. it's all off an 89 as well-just auto not 5speed. i changed big sensor on the throttle body (tb position sensor?) i think it may be my egr valve...or o2 sensor....i guess we'll find out after i hook the computer decoder up soon! ;) thanks!

derrktor feesh

90tbirdsc
04-15-2013, 11:45 PM
Could the vacuum lines on the back of the plenum leak 7-8 psi?

Thomas A
04-15-2013, 11:50 PM
Could the vacuum lines on the back of the plenum leak 7-8 psi?

The lines there would be leaking vacuum, not boost, as they are located before the supercharger. A boost leak would occur somewhere in the plumbing after the air is compressed.

Wow, this is an old post! hahaha

Thomas

90tbirdsc
04-16-2013, 12:09 AM
The lines there would be leaking vacuum, not boost, as they are located before the supercharger. A boost leak would occur somewhere in the plumbing after the air is compressed.

Wow, this is an old post! hahaha

Thomas

well at the beginning of the post Vernon said: Most of the time boost pressure leaks from the ends of old vacuum hoses that have gotten soft and swollen. Check the 3/8Ē hose connecting the intake plenum in the back, if it's easily pulled off by hand it positively leaks at 15 PSI. Under vacuum leaking fittings can pull together and seal up so it runs fine.

im just leaking A ton of boost and cant seem to figure out where its all going:confused:

Thomas A
04-16-2013, 12:11 AM
Have you confirmed it is actually leaking boost and not a belt slip issue?

Thomas

Vernon
04-16-2013, 12:11 AM
If you're missing that much boost I doubt you will find a boost leak that isn't obvious. That would be a sloppy fit on the cross-over hose or a loose intercooler pipe etc.. I would think that you'd have a vacuum leak from the same fitting.

If you're missing that much boost on the SC engine I would pull the intake elbow off and check the condition of the by-pass valve carefully. I once experienced a lack of boost condition near that magnitude that turned out to be caused by wear in the by-pass valve bore. Even when shut the valve allowed a significant amount of by-pass. Just my luck it was the one time I ever got the car on a dyno. The high pressure side vs. the low pressure side needs a good closed door seal to work. Once replaced I picked back up about 4 pounds. The particular piece had over 170K on it from use on two different engines. I used my SC's as daily drivers and then commuted over 40 miles each way in moderate city traffic. But as old as this post is Super Coupes are much older now. There's no telling what shape any particular unit is in without checking. Pull it off and work the by-pass throttle plate around. Look for excessive side to side play. You could rig up some sort of air pressure source on the high side and marvel at how much actually passes the closed by-pass. I know it's more than you would think.

Vernon

Vernon
04-16-2013, 12:14 AM
Ha ha, just over 10 years old. Wow. I still get emails whenever someone responds on my posts. I still have SC's but they haven't been ran in years now, about 5-6.

Vernon

90tbirdsc
04-16-2013, 12:19 AM
If you're missing that much boost I doubt you will find a boost leak that isn't obvious. That would be a sloppy fit on the cross-over hose or a loose intercooler pipe etc.. I would think that you'd have a vacuum leak from the same fitting.

If you're missing that much boost on the SC engine I would pull the intake elbow off and check the condition of the by-pass valve carefully. I once experienced a lack of boost condition near that magnitude that turned out to be caused by wear in the by-pass valve bore. Even when shut the valve allowed a significant amount of by-pass. Just my luck it was the one time I ever got the car on a dyno. The high pressure side vs. the low pressure side needs a good closed door seal to work. Once replaced I picked back up about 4 pounds. The particular piece had over 170K on it from use on two different engines. I used my SC's as daily drivers and then commuted over 40 miles each way in moderate city traffic. But as old as this post is Super Coupes are much older now. There's no telling what shape any particular unit is in without checking. Pull it off and work the by-pass throttle plate around. Look for excessive side to side play. You could rig up some sort of air pressure source on the high side and marvel at how much actually passes the closed by-pass. I know it's more than you would think.

Vernon
the bypass valve is in good shape, its very new I can pressurize one side and then open it and loose all pressure on the side that once had pressure. here is a video of my sc's leak. this is with a ported late model with a mp inlet at 15% od.

Video: http://www.youtube.com/watch?v=CH0PAlo7-BA&feature=share&list=UU_KBEm1RAchVUFGeEAj0i6A

here is the post I made about it but im not getting a lot of help or opinions:confused:

http://www.sccoa.com/forums/showthread.php?t=129673