Engine Hesitation/Stumble at WOT

[Felung]

I am having an issue with the the '86 stumbling under full boost above 4k rpm. I read up before writing this and have tried the following:

- inspect bellows for leaks or collapsing: all good
- replaced plugs (adjusted to .029), rotor, cap, wires
- checked timing: 10 deg w/o spout; 28 deg with spout
- Measured MAF voltage and seemed normal (did this yesterday and am having a brain fart right now and can't quite remember the numbers - but do remember them as normal from what I read)
- replaced fuel filter
- fuel pressure (measured from the valve on the fuel line next to the firewall)
- in driveway: 32psi idle, got up to 46psi revving engine
- on road: 32psi, moving to 50psi under full boost to 3500rpm, then slowly moving back down toward 40psi as rpm goes higher (seemed to settle about 42-44psi).

I recently replaced the ECT and that cured an idle issue I had been having and I don't know if that may have affected anything. At that time I also removed, cleaned and reinstalled the idle control valve (new gasket).

Engine runs perfect at idle and up 3500rpm then it is as if a the car just bogs/restricts/stumbles. It is not a miss of any kind. The boost is set to just in the mid "yellow" by the boost gauge and stays in the yellow while the problem occurs. The old plugs looked perfect, a bit grey to brown and in really good shape.

Other essentials on the setup, new exhaust about 5000 miles ago, 3" turndown and pipe with 3" cat going to dual 2.5's. A237, manual boost controller, cold air kit. Car has 65k miles original.

Your thoughts would be appreciated, very frustrating to drive without the fun factor working properly . I read a thread that mentioned fuel pumps, in tank and in line and suspect that as the culprit.

[NavySVO]

Vacuum hose to FPR maybe, or the FPR is going out.

[Nathan N]

FPR or intank pump. fuel pressure will rise 1psi for every 1psi of boost. base FP is 39 with vac line off. So it sounds like your gettting 15-17psi of boost and only getting 10psi of fuel raise. pull the vac hose on FPR when running and see if fuel is comeing out in the vac line. how long will it hold fuel pressure after being shut off?

[Felung]

Thank you for the feedback and I ran the fuel pressure tests with the vacuum line attached. So the 32psi to 50psi seems about right. I will check the vacuum line today and get back. Is the fuel pressure backing off after 3500rpm normal?

The pressure in the line stays at 32psi for a few minutes and then gradually goes down to about 10psi after about 10 minutes.

[Felung]

Resolved: Anyone for Operator Error?

OK, some simple things in life take so long to determine. I recently installed a 3G alternator along with an visible inline fuse setup that I purchased at West Marine. I noticed that the 150A fuse was blown and picked up another today. I then checked the voltage and it read 11.5 across the battery. I thought for sure the new 3G was shot because of something I must have done.....until I saw the main wire on the back of the alternator was lying loosely in the engine bay. The nut fell off (this is the operator error part) and is now reconnected and the car runs very well.

Thanks to Navy and Nathan for responding and for the others who have replied on the other threads which I searched extensively. One of threads mentioned electrical charging issues caused similar WOT problems.

[SVOeric]

so for once it wasn't the loose nut behind the wheel, it was the loose nut behind the alternator

[Felung]

New Problem - Battery Drain

Alright, Murphy's Law strikes again, I came back from Memorial Day vacation to a dead battery. I put a charge on the battery and it started and ran fine, within hours though, the battery is dead again. I had the battery load tested and it is fine. Might the fact that the main alternator wire was disconnected (see previous post, the nut fell off the main wire on the alternator and the wire was hanging loosely in the engine compartment. This also blew the 150amp inline fuse. I was running the car this way until noticing the windows weren't working properly.)?

I have a new 3G alternator and no regulator.

[Felung]

Didn't check the seat heater butt will do tonight...

....before the alternator wire fell off there was no problem with discharging but I can check tonight. I am ignorant on most things but especially eletrical, can anyone share how I can check if the alternator is pullling while the key is off?

I did some more searching and it seems that the check is done by test light or Ammeter by diconnecting negative battery terminal and connecting the ammeter in line between the terminal and cable, so I think I am good on that. If there are any other tips, please do share.

[Felung]

We I resolved one problem and another came back.

The problem was that the 3G alternator I just puchased went out when the nut came loose. The shop I purchased it from fixed it for no charge and all is good electrically now. I did notice that the WOT stumbling issue has come back though.

In the earliers posts on this thread I gave the fuel pressure numbers and I will repeat them again here (with vacuum connected):

32lbs at idle
50lbs under full boost, then the pressure drops to low 40's as the rpm goes above 4000. This doesn't seem normal. Can anyone confirm if the boost should stay at 50lbs through the rpm and what might cause this condition?

[Felung]

Thanks Mike, I thought the pressure should stay up and glad it's confirmed. I would imagine the pressure dropping at high rpm under constant boost creates a lean condition too. I purchased a fuel pressure regulator because it was mentiioned in another thread and it wasn't expensive and because my fuel pressure also drops from 32lbs to approx. 10lbs in about 5 minutes after shutting the engine off.

I have replaced the fuel filter and checked the vacuum lines all seems ok there. If replacing the FPR doesn't cure it, my next thought is the fuel pump. When I searched the stumbling issue before several threads implied there are 2 pumps, is that true and which would I check first if there is more than one?

[Felung]

Thanks for the updates. I just replaced the FPR and it made no change except in the size of my wallet. I will pull the tank and inspect the pump and hose over the next few days. I have a 1986 SVO with A237, 3" to 2.5" dual exhaust, manual boost controller, ported E6 and cold air kit. Should I consider a bigger flowing fuel pump or is stock going to be ok? I am not planning any additional mods.

[SVOeric]

I would repeat the fuel pressure test after fixing the alternator. if your alternator was not working, the fuel pump was running solely off the battery, while also running the ignition system, and any accessories you had on. It stands to reason that the numbers could be different with a working alternator vs a non-working alternator.

[Felung]

Thanks Eric, I will do that tomorrow. The symptom of stumbling after 4000rpm is the same as before though, but connecting the fuel pressure gauge is easy enough so worth a try.

[BlueCASVO]

I just got this exact problem fixed on my 85...turned out to be alternator/regulator/battery. I took the alternator in, which tested bad, so I replaced it. Problem persisted, so I replaced the regulator. Then the battery wouldn't hold a charge, so I go t a new one of those, as well. STILL had the problem, so I took the new alternator in, which ended up being bad. So, I replaced the alternator, checked the new regulator (which now was BAD) before I hooked it up, and replaced it also. Now it works just fine.

I guess that when one of those three components goes bad, the other two follow quickly. I know you are running a 3G, but if your fuel pressure and ignition are set correctly, I would think it would be charging system still...

[Felung]

Blue - thanks appreciate the reply. By chance did you check the fuel pressure? The thing that I don't understand is decrease in fuel pressure when the boost stays constant.