Shudders on acceleration @ 5# boost

EZBird

Registered User
We just had the pleasure of doing the head gaskets, they had gone suddenly and engine was taking on water and usual symptoms. (113K miles on engine but we found someone has been there before!)

When we started the engine it smoked for a while and we idled it until it quit smoking. Once coolant was completely full we have no more loss. We flushed the oil after about a 10 minute idle. Engine is idling very smoothly and we did put in new platinum plugs.

Problem is on road test it shudders, stumbles at just above 5# boost. If you dont use any boost it runs perfectly smooth- I'm guessing something to do with SC. The car did not have this symtom before the head gasket(s) went.

When it shudders or stumbes you can see the boost guage drop. IS there something we could have hooked up backwards to cause this?

In the driveway with a warm engine you can advance throttle slowly to 4000 RPM without any stumbling or hesitation - it maintains about 20" of vacuum. It's always on road tests when the boost goes to just above 5# when the juddering, missing?, starts and the needle drops back.

There are so many vac line and plugins, it is possible we created this problem. We tried to tag everything and return it to the correct places but may have slipped or interchanged one vacuum hose. Most electrical plugs only fit one place.

Does this sound familiar to anyone?

Thanks.............
 
Check the fuel preesure regulator, there is a short hose the goes from the inlet plenium to the regulator that opens the regulator up under boost. Sounds like your not getting enough fuel to support boost. Or leaks around th I/C tubes could cause some troubles.
 
Sounds like you put in the wrong plugs. I had this problem a couple of times. either bad plug wire(s), or wrong plugs.
 
Check the connections at the intercooler tubes. (or just re-seal them with lots of blue rtv).

How's your vacuum at idle (low? like 6-11)
 
The new plugs we put in were AP2546 which is the same as we removed. The car ran great the previous 2 years with those installed.

Plug wires were meticulously routed and confirmed during reassembly and while they could be seen.

Not sure of idle vacuum but you can easily drive the car and maintain 10-20 inches of mercury for vacuum. You can increase RPM's to 4000 slowly and manitain 20" Vac (in driveway) Driving indicates a good running engine unless you put your foot in in.

Engine sounds and runs very smoothly until you road test and try to use more than around 5# boost. No loss of water, oil stays very clean with normal pressure, temp is at mid guage - normal.

Haven't found any hoses disconnected but we are not really sure if they all got back to their original location. The ones on the SC and the Air Horn are th hoses most suspect as it was a wrestle to get those and the SC out.

Also the 2 hoses we disconnected at the DR firewall when removing the WWipers assy. Can't figure where vacuum gets involved with wipers but the way we have things both the wipers and washers work- now if the SC would, or the engine would run boost OK we'd be more happy.

Does anyone know of good documentation for the vac hose routing on a 91?
 
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Check your balancer for damage. The load from the engine not running on all cylinders may have caused your balancer to break and throw the timing off.

Also check to make sure that your fuel regulator has the vacuum line connected, this will get you the increase in fuel pressure when under boost.

Jerry
 
I am now having this problem with my 95 after putting everything back together. But I am gettin around 8 - 3 hg vacuum, a very bad miss, engine is shaking horribly, almost like there is a huge cam/heads. Ive checked all vacuum lines, ic connections, connections on the chip/ecm, plugs and wires. I'm stumped. I'm gunna try swapping my plugs next. Keeping in mind that all ran smooth before disassembly, I've only done the following mods in between. RB manifold/return plenum, underdrives and bhj, magnecore wires, and chip. I'll let ya know if come across something, as I'm now in your boat.
 
Coil pack - plug lead ID's

The vacuum line to the fuel regulator is connected and all other vacuum lines check OK. I found one a bit loose but trimmed and its tight now.

Everything is very smooth until boost occurs- still. Engine idles with vacuum of around 17-20 -depending on idle rpm's.

Got new plug leads and have installed the drivers bank so far. The plugs were new so for now shall assume they were not DOA or breaking down under load.

Does anyone have or know the order of the coil pack position to cylinder number. I can't find any docs to confirm this order. I'm attaching a drawing of what we figured and have used. To our best knowledge this is the way it was on dissembly.

We only changed the head Gaskets and the plugs so we're wondering how we could have introduced this problem, unless we wrecked a plug lead removing them.
 

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Put the car in Park or neutral (750 rpm) and with the engine hot you shoulkd have 20 In Hg of vacuum. Don't go by the factory guage, get a good mechanical gauge and install it on the lower intake tube where the factory gauge hooks up.

You must determine whether or not you have good vacuum. If you have a vacuum leak this will cause a lean condition under boost. If that checks out then you can move on.


If it was me, I would buy a set of Motorcraft 8mm stock wires and replace the ones you have. You wouldn't beleive the misses at high rpms and load a regular engine can have with bad wires. A boosted engine has much higher cylinder pressures than a regular engine and good wires are a must.

There are a lot of things Ford did sort of halfassed on the SC, but the ignition system is really pretty strong. Factory plugs and wires work very well even on heavily modified SCs.

Keep us informed, you are correct on your coil pattern.

Jerry
 
Jerry thanks for the info. We have already installed new Bosch 8MM wires on the drivers side bank, so we'll have to stay with those now.

The plugs we installed were the same as what ran well before, but I never checked if they were the correct type for this application - they are Autolite Plat AP2546.

A quick trip around the block indicated improvement, but I won't be convinced until after a good run with engine properly warmed. It should be easy to see when its gone, it was a hard fault, easy to produce.

I'll get a vacuum guage and check on the line where the vacuum goes to the factory guage, if thats where you meant.

I'll post when we get further along or have some positive results- thanks again for your help...........Rick..
 
Here's a vac dist diagram from a 93-should be very similar to yours:

258979_67_full.jpg
 
Pearl & EZbird
You could try and push a wire into all the vacumm connections on the plenum.And make sure they are not blocked.Then put a clamp on every vacumm line you can get to on the motor.Uses plastic ties on the small vacumm lines or ones hard to get to.Some of the vacumm lines will leak under boost.You may not think they leak.But old rubber vac lines will leak under boost.Vac should be 19 to 20 lb.

If clamping the vacumm lines don't work.Check plugs and wires again.Clamping the vacumm lines will help performance.Even if this is not your problem.its worth doing.

Randy
 
We'd like to be sure it is a plug or wire or vacuum leak causing the miss when the engine goes into boost. We're also low on time to do repairs at present but could use to put some easy miles on the car.

Is it safe to remove the SC belt and run that way for the present? Will the engine be OK with no damage? Currently it has some hard misses when you get into boost.

If the car runs OK w/o th SC it may prove the problem is related to the boost and not some other electronic part.

We plan to follow up on replacing the pass. bank plug wires ASAP and also to follow advice offered re vacuum leaks. The post by Vernon on the effects of boost on plugs, wires and also how to chase down the leaks was most informative. Surely justifies why you guys are all saying - plugs and leads and vacuum.

Question comes to mind after reading that post - do the vacuum lines turn to pressure lines under boost along with PCV valve etc or are there 2 parts to the vacuum circuits.

I also notice some "plastic devices" in those lines going to the rear of the SC and Air intake. What are they about and do they ever fail?

Any ideas?
 
The plastic devices on the vacuum hoses are check valves....I think you have a bad plug or plug wire. Here is a diagram with the correct plug locations.

PlugandWireLayout.jpg


David
 
Is this mystery unfolding?

The new plugs we installed were Autolite AP 2546 which were identical to the ones removed. Prior to the HGasket job the car ran fine under boost, currently it misses and shudders under boost. It runs fine with no boost.

From the Autolite site I see the plugs should be AP 2544 - unless that listing is incorrect for this application. 8th digit of VIN is R.

Does anyone know the difference between these two plug types and if the ones we installed are correct or what the heat range difference is.

What plugs are you guys using? Has anyone had this experience? The last plugs were installed by a Ford dealer. I'm having doubts on this and it's easier to confirm prior to changing these in place.

Could the new Head Gaskets, FelPro, be creating more compression and thereby causing these plugs to fail. (under boost only - due to heat range?)

I'm not familiar with, nor do I have any documentation on heat ranges for the autolite platinum plugs. Strange thing is, the car runs better under some boost before it warms up fully, then it gets worse. Engine temp is normal though.

Any insight to stop shotgunning this problem is appreciated.
 
What Plug works best?

No progress to date, we've been away. I'm wondering if anyone has any good words on this.

Surely it would be best to just put in then most reliable plug with correct heat range. What are you guys running for plugs in stock engines?

Any insight is appreciated. Thanks....Rick.
 
I had the same problem you are having it turned out to be 2 switched wires check the diagram posted earler in this thread also veryify that no plugs were cracked on installation
 
Double platnium plugs

You should really be running double platnium plugs. This is the correct autolite part number for the stock plug.

Autolite APP2544

The PP designates a double platnium plug. I think you probably have a pug wire swaped. When I did the motor mounts I changed my plug wires and I had similar problems. I though for sure I had it right, and I did on the DIS, but I had the 4 and six wires swapped on the block. Put em straight and fixed all my problems.

Dave
 
Big Cat Davo said:
You should really be running double platnium plugs. This is the correct autolite part number for the stock plug.

Autolite APP2544

The PP designates a double platnium plug. I think you probably have a pug wire swaped. When I did the motor mounts I changed my plug wires and I had similar problems. I though for sure I had it right, and I did on the DIS, but I had the 4 and six wires swapped on the block. Put em straight and fixed all my problems.

Dave

While many think otherwise, I've learned that running double platinums isn't necessary. Running copper plugs won't affect performance noticeably. Even running 1 Autolite, 3 Motorcraft and 2 Champion :)

I think you're barking up the wrong tree worrying about the plugs themselves.

Edit: By "noticeably" I mean when new.
 
Plugs

Copper is fine, they just need to be changed more often. But single platnium is no good due to the way the SC ignition fires. It can lead to missfires under boost.

Dave
 
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