Nose over at WOT

hytorksc

Registered User
I have been searching and reading ALOT of posts here about SC's having hard hesitation when over 5psi boost at WOT. I now have this problem. I have diagnosing this for over 2 months on mine, and at some points I had thoughts of getting out of the SC period. But I stuck to my guns, and after replacing just about every electrical sensor/ignition part on this thing (including wires/plugs), I finally get the urge to move on to fuel system. I connect a FP meter to the schrader valve on the fuel rail, and read it as I drive. Here's what I found;

idle pressure = 37-40psi
0 to 5psi boost = 40-45psi
past 5psi at any rpm = 20-22psi

in other words, when I floor it, the meter goes up to 45psi until boost goes past 5psi, then it drops immediately to 20psi.

I know this means fuel pressure reg or fuel pump, but how can I tell which one?

It's not likely that the fuel pump would just drop like that at the same point each time would it? Besides the fuel pump is a recent install (190lph). I even put a jumper across the inertia switch in the trunk just to make sure it was not a continuity problem there- but it did the same thing.

When the fuel pressure reg is in vacuum at idle, is it supposed to limit the fuel to 35-40psi, and when you go WOT with no vacuum it lets more pressure thru at 50-55psi assuming 15psi of boost? I tested this with the FPR vac unplugged, and it still nosed over at WOT at the exact same point. Could this mean that it's not the FPR?

I'm thinking a fuel filter or crimped line would not do this because it suddenly drops pressure rather than gradual. One thing I did notice is that when I unplug my alternator the fuel pump sped up slightly- I even checked that and it's reading 14 volts which is good.

The car idles and drives perfect until you try to get on it over 5psi boost. Very weird.

I don't get this at all- sorry about all the crazy questions, but I am inviting any and all opinions here. Maybe we can settle this problem once and for all on the SC- I know alot of you out there are having the same issues.
 
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Make sure you don't have a leak in the vacuum line to the regulator and check for gas in the vacuum line.

Most likely you have a restriction in the fuel line or you have a bad fuel pump though. Change the fuel filter even if it's relatively new.

Good job so far pinpointing the problem.
 
XR7 Dave said:
Make sure you don't have a leak in the vacuum line to the regulator and check for gas in the vacuum line.

Most likely you have a restriction in the fuel line or you have a bad fuel pump though. Change the fuel filter even if it's relatively new.

Good job so far pinpointing the problem.
thanks-

the fuel pump is new (190lph), but it is whirring a little loud for some reason. I noticed that it does put out close to 50psi at 5psi boost, but beyond that it nose dives to 20psi very suddenly. would a bad fuel pump typically do that? The pump didn't sound like it slowed down at all when the fuel pressure nose dived.

I looked under the car and traced the fuel line from the lower part of the tank up past the firewall, i didn't see a crimp anywhere. I will need to verify the line going over the top of the tank and into the fuel pump itself (will have to carefully cut an access hatch under the back seat on the left side- done this before sucessfully on a prior SC- just have to be very careful of the wires back there. Probably will do some good to change the fuel filter as well.

I looked very hard under the hood and i did find a big boost leak on the crosstalk hose (a large hidden crack) and guess what- coming off the return plenum right next to the FPR hose, which could possibly make a difference in how the FPR works- i mean when vacuum turns into higher boost, the hole in the hose expands blowing out boost and creating a reversion to the vacuum side at the FPR (?). The car was running fine at WOT a couple months ago, and i do remember putting small clamps on these hoses- calling myself sealing up everything. Instead, i think i just missed this hose (or unknowingly broke it) for some reason.
 
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Well, loss of fuel pressure can only be caused by 2 things. 1) lack of available fuel (blocked lines or bad pump) or 2) regulator not holding boost pressure.

In the case of 1) when engine load gets to a particular point fuel demands exceed supply and pressure drops. You will find that 5psi at 2000rpm demands a lot less fuel than 5psi at 4000rpm so test to see if the drop in fuel pressure is strictly boost dependent or if it is tied to fuel demand (rpm) as well.

In the case of 2) check the FPR line for traces of fuel. If the diaphram is bad then fuel will creep into the line. Check the line itself for possible leaks. You might replace it with a section of new vacumm line in case the plastic is split and you can't see it. Because fuel pressure is tied to boost, then for fuel pressure to drop to 20psi, then you'd have to be seeing a vacuum of probably 35 to drop fuel pressure that low. Even if you removed the vacuum line completely from the FPR you'd see a constant 39-40psi fuel pressure.

I put money on lack of fuel due to blockage or perhaps the fuel pump has partially blown off it's output line.
 
XR7 Dave said:
Well, loss of fuel pressure can only be caused by 2 things. 1) lack of available fuel (blocked lines or bad pump) or 2) regulator not holding boost pressure.

In the case of 1) when engine load gets to a particular point fuel demands exceed supply and pressure drops. You will find that 5psi at 2000rpm demands a lot less fuel than 5psi at 4000rpm so test to see if the drop in fuel pressure is strictly boost dependent or if it is tied to fuel demand (rpm) as well.

In the case of 2) check the FPR line for traces of fuel. If the diaphram is bad then fuel will creep into the line. Check the line itself for possible leaks. You might replace it with a section of new vacumm line in case the plastic is split and you can't see it. Because fuel pressure is tied to boost, then for fuel pressure to drop to 20psi, then you'd have to be seeing a vacuum of probably 35 to drop fuel pressure that low. Even if you removed the vacuum line completely from the FPR you'd see a constant 39-40psi fuel pressure.

I put money on lack of fuel due to blockage or perhaps the fuel pump has partially blown off it's output line.

When I removed the vac line from the FPR, it still nosed over at 5psi boost, and the FP dropped from 40psi down to 20psi. So this means the problem is at the fuel pump or somewhere in the line.

That's too bad, because the pump was a fairly new install (the guy i bought the SC from did it). I will have to do it over again with a new pump. Before I do this I may try hard wiring the pump using a relay just to verify if it's an electrical circuit problem- it could be cutting out at that 5psi point for some reason. The circuit that controls the pump when you do a KOEO may have something to do with it- i just don't know.
 
Try your experiment with a FULL tank of gas maby its your pick up and when you floor it the gas in your tank sloshes backward and the pump sucks air.
 
this is probably just a waste of time, but could everything be working just fine, and he's getting belt slip at roughly 5 psi because of a bad tensioner or a worn belt? could the sudden slip cause a pop of the boost recirculation valce or whatever it's called that would cause the boost going to the fuel pressure regulator drop out or even jump into vacuum for long enough to cause the fuel system symptoms? Does it keep producing the 5 psi with slower than expected acceleration, or does it stop producing boost at all? Maybe there's a leak in the IC tubes that only occurs at 5 psi and then the engine gets flooded because the computer still thinks all the air from the MAF sensor is getting to the engine when in reality it isn't... so now the computers firing the injectors at the faster rate it would need if all this air was there, causing a pressure drop in the fuel line...hence why you still see the drop with the vacuum removed?

crazy thought, but normal logic hasn't found it yet, so it can't hurt to think outside the box.
 
ThunderCoupe said:
this is probably just a waste of time, but could everything be working just fine, and he's getting belt slip at roughly 5 psi because of a bad tensioner or a worn belt? could the sudden slip cause a pop of the boost recirculation valce or whatever it's called that would cause the boost going to the fuel pressure regulator drop out or even jump into vacuum for long enough to cause the fuel system symptoms? Does it keep producing the 5 psi with slower than expected acceleration, or does it stop producing boost at all? Maybe there's a leak in the IC tubes that only occurs at 5 psi and then the engine gets flooded because the computer still thinks all the air from the MAF sensor is getting to the engine when in reality it isn't... so now the computers firing the injectors at the faster rate it would need if all this air was there, causing a pressure drop in the fuel line...hence why you still see the drop with the vacuum removed?

crazy thought, but normal logic hasn't found it yet, so it can't hurt to think outside the box


Wouldent he see that as a jumpy boostgage?
 
Normal idle vacuum is ~20". This drops fuel pressure from static 39psi to about 32-34psi. For fuel pressure to be pulled as low as 20psi vacuum would have to reach a monstrous value which is not happening especially at 5psi boost. My opinion is that it is starving for fuel due to blockage or a fuel pump issue. ;)
 
Check for damaged fuel line or filter. Surely the guy installed a new T-Bird fuel sock on the new fuel pump. Most 190lph walbro kits are for Mustangs and they use a different fuel pickup sock. Fuel line can also get pinched when the fuel tank gets reinstalled.
 
My IC tubes are sealed up pretty good with the red RTV silicone stuff. But as i stated before, i did find a major leak (crack) on the crosstalk hose on the inlet and return plenums. Actually the boost gage seems to hang at 7-9psi after it has nosed over in 2nd gear at about 3,000 rpm, but if i hold it in 1st, it will go to 13 psi, and still nose over hanging at 4,000 rpm. The nose over event starts as the boost passes 5psi. The boost gage is not erratic at all, but it does go back a little when it noses over. The engine does behave like it's severely fuel starved or partially cutting out- but with no detonation- very weird.

I modded my SC tensioner to put more pressure on the supercharger belt (this was one of the first things i did before i posted here). No evidence of slipping (black powder) anywhere. I've been thru all the basic diagnostics and replaced virtually every sensor, coil pack, dis, maf, including autolite 2544 plugs and new wires before posting here (no cracked plugs either). The only sensors i did not replace were the crank and knock sensors. The problem would not be the crank sensor because i had just got a new harmonic balancer, and the nose over problem was not happening right after this was done. I even disassembled the crank pulley and made sure the crank sensor was not rubbing the vanes on the HB. This problem is most definitely related to what XR7dave is saying. However, i cannot rule out the possibility that something's happening "outside of the box" here.

Right now i am diagnosing electrical continuity with the ignition on between the fuel pump and the relay. The only thing i haven't found yet is the fuel pump relay. Is that in the IRCM box?
 
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Shockwave said:
Check for damaged fuel line or filter. Surely the guy installed a new T-Bird fuel sock on the new fuel pump. Most 190lph walbro kits are for Mustangs and they use a different fuel pickup sock. Fuel line can also get pinched when the fuel tank gets reinstalled.

The guy who sold me the SC actually was getting into mustangs which leads me to believe he may have put a mustang 190lph walbro with that fuel sock. He may not have installed it correctly because I had to follow up his work on alot of things when i first bought the car. i think i'm just going to redo the whole fuel system up to the rails.
 
ThunderCoupe said:
probably, but if he's looking at the boost gauge, how is he looking at the fuel pressure gauge?

I had the fuel gage taped upright in front of my windshield so i could see both. but you're right- i cant look at both at the same time, but i took very quick glances at both when it was happening, and read them as best as i could.
 
the gas sloshing...isnt that what a fuel cell is for?
 
I wish I could have back the time and headaches I've wasted on what "he said". My point is you have a fuel restriction problem without any doubt. To just assume you been told the truth can cause many headaches. Is it a 190lph? Is it "new"? Is the fuel sock the correct one?
Here's another scenario. The guy replaced the fuel pump, found out he had the wrong fuel sock, so he puts on the old original one back on. When I did my fuel pump there's no way I would've used the old sock because it was partially clogged at 100,000 miles.
You need to drop that tank check/replace the fuel pump and sock with known quality. Then check each section of fuel line for kinks or obstructions. I'd also replace the fuel pressure regulator to eliminate that issue. You also mentioned the fuel pump was noisy. My opinion is a noisy pump is a failing pump. Good luck, John
 
AnnivSpeCpe said:
I wish I could have back the time and headaches I've wasted on what "he said". My point is you have a fuel restriction problem without any doubt. To just assume you been told the truth can cause many headaches. Is it a 190lph? Is it "new"? Is the fuel sock the correct one?
Here's another scenario. The guy replaced the fuel pump, found out he had the wrong fuel sock, so he puts on the old original one back on. When I did my fuel pump there's no way I would've used the old sock because it was partially clogged at 100,000 miles.
You need to drop that tank check/replace the fuel pump and sock with known quality. Then check each section of fuel line for kinks or obstructions. I'd also replace the fuel pressure regulator to eliminate that issue. You also mentioned the fuel pump was noisy. My opinion is a noisy pump is a failing pump. Good luck, John
thanks-

Yep, i'm going to just replace the whole fuel system- if there are no crimps in the fuel feed line at the gas tank, i'm not going to worry about that. First i'll try fuel pump/filter/regulator and see what happens.
 
I found the exact problem

The fuel pump (btw, i verified it as a brand new walbro 190lph- so the guy didn't lie to me about this at least) has a "mini collander" that fits at the intake of the pump to keep debris out of the pump rotors. this collander is between the pump inlet and the fuel sock. believe it or not, it had big clumps of black soot on it- so much that it blocked 90% of the flow. it's a wonder how that engine idled at all. There is also a tiny pintle needle valve with a spring at the fuel outlet of the pump motor which closes up when the pump is off to stop backflow and maintain some static pressure at the rails when the engine is off. i took it off and found that it was sticking a little. i cut a small amount of the spring off to reduce tension a little and reinstalled it. i think what was happening was when the clogged collander at the pump inlet restricted enough fuel flow, the pintle valve at the outlet would lose pressure and close up, cutting off fuel, which would explain the nose over at 5psi boost. i finished making the changes/fix, cleaned up the entire pump and reinstalled it in the car. restarted the engine, got the static pressure back up to 40 psi, and about 38 psi when idling.

i then sealed up my remaining vacuum leak, and took the car out for a test.

Guys, the difference this change made was absolutely phenominal.

Not only did i get a solid 14+ psi boost with no nose over, the car can now spin the tires all the way thru 1st AND 2nd gear on an aod with stock 3.27's(and i have sticky kuhmos). it has never done that before- in fact i never had a mild modded SC that was able to do that.. ever. while spinning the tires it hangs out at 4,500rpm with no bogging or hesitation. accelerating from a 40 mph dig is now effortless. 0 to 60mph is virtually instantaneous. if you have seen a WRX STI launch hard, that is how my car launches except for the traction losses. it feels like high 13's now.

i think that this is the key for other SC's that are having the same problem with no apparent solution assuming everything else is working correct.

Bear in mind that this may only apply to the 190lph walbro fuel pump, i'm not sure if this fix works for the stock pump (i would think it's similar though).
 
Glad to hear you got it fixed...and thanks for telling everyone what the problem was. Very often people ask for help to fix a problem and never report back with their findings.

David
 
David Neibert said:
Glad to hear you got it fixed...and thanks for telling everyone what the problem was. Very often people ask for help to fix a problem and never report back with their findings.

David
thanks David.
 
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