Scott at Team Harco

My girlfriend's 85.5 had a fuel leak (at the pump) that we fixed yesterday. The engine has been running rough - intermittently. We were hoping the leak would somehow be related to the rough running engine. Evidently, this is not the case.

Here is what we know and what we have checked:

1) Engine will start and run at idle, just fine.
2) More and more frequently, at RPM above idle the engine will run rough.
3) It sort of runs like only three cylinders are working - but all plugs and injectors have been checked and they seem to be working fine.
4) Compression check: all cylinders 140-145 psi.
5) Swapped MAF and the same problem exists.
6) Checked fuel pressure at the fuel rail: ~34 psi at idle. Slightly less at higher RPM, with rough running.
7) Fuel pressure fell off to about 10 psi within about one minute of shut-down.

Plan is to swap out the fuel pressure regulator next. If that doesn't do it - we'll swap the DME.

Anyone else have ideas on what is wrong or what else to look at?

Lizard944

Bad injector. Get an infrared temperature gun at Harbor Freight ($30.00) and check the exhaust manifold tube temps. The one with the cooler cylinder is the offending injector. You're looking to find which cylinder is missfiring to narrow down the problem. It will be 50-100 degrees cooler than the rest. Not just a few degrees. Try giving the injector a few good taps with a socket extension and hammer (there is a youtube vid on this) while the engine is running. This will sometimes free up a sticking injector until you can replace them.

JohnKoaWood

http://www.clarks-garage.com/shop-manual/fuel-06.htm

at least test it... on N/As it is ON IDLE / OFF IDLE only...

Also test the injectors (pull bolts, lift up rail with injectors attached, crank motor to verify all 4 are squirting..)

Definitely sounds like you either have a bad TPS, bad fuel pump check valve, or restriction in the line (fuel filter change????)

Scott at Team Harco

I was thinking TPS would be a possiblity. Will check that when I have time to mess with it again.

Injectors are all working. How well - I don't know. I pulled each connector, one at a time, to see if the state of operation changed. Each injector did alter the way the engine ran, as each was disconnected and then reconnected. I think there is a different and bigger issue.

Filter has not been ruled out, but we put a new one on when we got the car two years ago.

We'll figure it out. Keep the ideas coming until we do.

Scott at Team Harco

Update from 8/1.

It is not the fuel pressure regulator.
It is not the TPS.

Still not fixed...

adrial

Does load have an effect on how rough it runs?

Mark944na86

High-tension arcing at higher RPM? When was the last time you changed sparks plug leads, cap and rotor?

Scott at Team Harco

It does not run right even without load.

After swapping the parts I swapped out yesterday - it wouldn't even idle right. I was reminded (by Dr Goodwench) that we experienced this previously. We were starting and shutting off the engine in rapid succession when we were "burping" the power steering system. There were random cases where the engine did not run right. Again - random. It would run right after shutting down and letting it cool for a bit (not sure if it is a temperature dependent issue, or not).

Scott at Team Harco

Cap and rotor are fairly new (about two years). Plug wires have never been changed, as far as we know.

I do have a new set of plug wires. Will look at that tonight. I did a spark check with an in-line light. That did not reveal a problem. However, I can use an inductive timing light to check again.

Will also have an infrared temperature reader at my disposal tonight. Should be able to detect temperature differentials at each exhaust outlet.

Should have more pieces to the puzzle after tonight.

SamGrant951

the quick drop in fuel pressure sounds like the fuel pump check valve but I dont see how that would cause the running issues described.

Scott at Team Harco

More trials (8/2)

It is not one of the injectors.
It is not one of the plug wires.

I thought I was getting a cooler reading off the #3 exhaust port. I swapped the injector and a plug wire from Herbie. No difference on Penelope. At first I thought it was fixed when it ran well at idle. As soon as I went off idle, it ran like crap again.

With the #3 injector on Herbie (from Penelope) there was no change in the way it ran.

Next trial will have to be the DME.

Scott at Team Harco

Update from 8/12.

Have messed with injectors and damaged a couple of o-rings. Swapped out good o-rings from Herbie and put it all back together with no leaks (hurray!).

Car still runs fine at idle. It will also run well as revs increase. Once throttle is reduced or brought back to idle, the car runs like crap. WTF!

Swapped out the DME yesterday. That made no difference. It's good to have a running parts car to do direct A-B and B-A comparisons. However, this is getting very annoying not knowing what path to take.

Dumped some Seafoam in the fuel tank on the odd chance it might help. No obvious improvement.

Next up on the list will be the fuel filter and fuel pump.

Still accepting ideas...

86 951 Driver

Just throwing this out there. How rough is it running? I would do a compression check. Maybe a leaking headgasket?

I would change the fuel filter and also run that seafoam through and see if that improves at all.

Far shot. Fuel Dampner?

Trying manually cleaning the injectors like described in Pelican Products tech articles.

Scott at Team Harco

Compression check was performed. All is good.

Will look at fuel filter and fuel damper. I'll have to check Pelican - but I don't think that is it.

Scott at Team Harco

From 8/15:

Fuel pump and fuel filter are not the problem.

Fuel damper is not the problem.