Car hesitating at WOT

sleeper

Former LOD President
My '93 is hesitating, almost as if it's out of gas, whenever I go WOT and let the revs get up there a bit. Sometimes it happens at low revs and low throttle openings too, but it's more consistent up top. The rest of the time it runs pretty well, except it occasionally stumbles right off idle too.

I changed the fuel pump recently, (car wouldn't start + no fuel pressure) and now it runs but does this. I checked fuel pressure while driving it and I lose a little bit of fuel pressure when this happens but not nearly enough for that to be the cause. I changed the fuel filter anyway and that didn't help. I need to hook the gauge back up and look a little more closely (hard to read the gauge in the dark) but it doesn't look like that's related to the problem. It loses power hard, as if the engine is about to die, and it would have to lose almost all fuel pressure to do that. The fluctuation I was seeing was maybe about 5 psi, something that could be affected by vacuum somehow. But the car doesn't appear to have any more vaccum leaks (I fixed all those when I bought it).

No codes KOEO or KOER. I have a pimpy snap-on scan tool on long-term loan from a friend, and it said nothing is wrong. But something ain't right. There was a stored code in there about a CID sensor, but when that failed on my '94 it did the stumble on tip-in when I first started it but it didn't give me any trouble after that. Now I have that problem plus the stumbling when I'm on it. And the code went away when I scanned it again.

Can LOD help me out?
 
Check the fuel pressure regulator. Especially look for fuel in the vacuum line on top. The diaphram goes bad and leaks fuel to the vacuum side at times. Double whammy of raw fuel in the intake and the same effect of low fuel pressure due to the leak.

That's one guess.
 
Check the fuel pressure regulator. Especially look for fuel in the vacuum line on top. The diaphram goes bad and leaks fuel to the vacuum side at times. Double whammy of raw fuel in the intake and the same effect of low fuel pressure due to the leak.

That's one guess.

+2.
 
This sounds like a problem I used to have with my '94, but only after the car had been running for 15 minutes or so, IIRC. (It has been 8 years since.) I traced that problem down to a bad fuel injector o-ring since rubber contracts when heated. They may be worth a check as well, largely because they are cheap to replace. Just be VERY careful with the injectors; they are NOT cheap to replace.
 
The fuel pressure regulator looks fine. No evidence of fuel in the vacuum line and no smell of fuel in the vicinity. I hooked the pressure tester back up, and when i pulled the vacuum line off the regulator, the pressure went up 10 psi or so. So it looks like the regulator is working fine.

I think this is probably not a fuel problem.
 
Went up a bit when I checked it. But the gauge acted a bit funny afterward so I need to do it again. I'm thinking it's probably not a fuel problem, but I don't know.
 
Tape the gage to the winshield and have another set of eyes read the gage as you drive the car around to put it under load.
 
Well I finally got around to diagnosing this problem. First thing I did was replace the "CID" sensor (actually a cam position sensor) as I did have a latent code for it at some point. I also replaced the wires leading up to the pigtail as that was the actual problem on my '94. I think that took care of the problem with the car hesitating right off idle. That was the problem / symptom in my '94, and that seems better based on a short test drive.

But the problem with the hesitating at higher throttle openings is still there, and it IS a fuel problem. Once I go the POS autozone fuel pressure gauge working right I saw fuel pressure drop whenever it hesitated.

Fuel pump is new. Filter is new. Any way to check regulator besides just tossing the part at it? Hopefully the one I have on my '97 fuel rail will work.

Another suspicion is the pump, precisely because it is new. Pump died, car wouldn't run. Replaced pump, car runs but does this. Coincidence? Maybe, maybe not. Any idea how the pump could cause a symptom like this? With a return system I wouldn't think it likely.
 
I remember that same issue with my '94 as well. I replaced the sensor, and as I plugged the connector back on, the wire leading into the harness snapped from corrosion. Have you checked out those injector O-rings yet?
 
No, but those wouldn't cause low fuel pressure, would they? That would just be another vacuum leak.

I'm half tempted to throw another fuel pump at it. This time i'll cut the floor.

I looked at the FPR. The gen 1 and gen 2 are different. And the gen 1 looks like a bitch to replace. Fuel rail has to come off.

What kind of failure mode would cause the fuel pressure regulator to intermittently drop pressure with the throttle open?
 
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The more I think about it the more I think fuel pump. I just can't think how a fuel pressure regulator failure would cause the symptoms i'm seeing, and the fact that it showed up after a fuel pump repair is just too big to ignore. I'm going to have to inspect the tank to make sure no vent lines got crimped when it got put up (roommate did it for me) and if that looks good i'll swap the pump again.

When it's not hesitating, fuel pressure does what it's supposed to, goes up with less vacuum and down when I close the throttle. When I open the throttle and demand goes up and pressure starts to rise it's like the fuel pump can't keep up, as if the flow isn't there and the pressure drops as a result of that.

So I guess i'll throw another fuel pump at it and report back. I'm not sure when that'll be though, as Lemons is in 2.5 weeks and my waverunner broke again.
 
If you have a vacuum leak attached to the combustion chamber, then I could see how it would reduce your overall fuel pressure...
 
The injector isn't connected to the combustion chamber. It's positioned in the intake above the valves. Failure of the top o-ring would cause a fuel leak, failure of the bottom o-ring would cause a vacuum leak.

In any case - the car had some MAJOR vacuum leaks when I bought it, and it didn't do this. I would not attribute this to a vacuum leak. And I did look around the fuel rail, it's not leaking fuel either.
 
Well I replaced the pump again and it's still doing it.

All that's left is the regulator and the relay. I'm going to go buy both and replace them. In the process I will look at the injector o-rings, because the fuel rail has to come off anyway.
 
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Regulator didn't do it. Relay wasn't in stock. I really don't think it's the relay, but I don't know WTF else it could be. Only way I could see the relay causing it would be a voltage drop across the relay, and that's really not a typical failure mode for one of those.

All the top o-rings look good on the right (new since I lost some), on the left they're not leaking but I didn't pull up that side of the rail. Bottom o-rings aren't showing any signs of leaking either but that would just be a vacuum leak anyway.

WTF else could cause pressure to drop under load? Doesn't do it at low load. Only does it when I open the throttle. Doesn't seem RPM dependant at all. And it only does it momentarily. If I floor it, it'll start to rev, drop fuel pressure, damn near stall, then if I keep my foot on the gas, pressure comes back and it'll rip right up to redline. Sometimes it'll do it twice on the way through the revs, sometimes only once.
 
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Will it hesitate with a full tank of fuel? Might be the empty basket syndrome that happens to people that use the 255 pumps.
 
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