Stumbling/hesitating, running out of options

[arcteryx]

Need some assistance on this one as I am exhausting my ideas/options. My 94 runs like crap and appears to be missing and I think it's something odd based on what I have done below.

Symptoms
- when driving there is noticeable stumbling and hesitation. I do not want to drive it. Idle it is not noticeable when in P, but is when in gear
- oil pressure gauge is high on start up, normally it's low. Could be a bad gauge as they are prone to error

Things Done
- At first the thought was the spark plug tube seal in the valve cover was so old brittle that oil was coming into the tube and fouling up the plug. This was true for #3. I replaced the tube seals with new ones
- Tune up - new plugs, wires, cap, rotor, valve cover gasket
- removed plenum and replaced all hoses with new OEM hosing. Tested all VSVs which were all working
- fuel pressure was tested and passed by my indy. I did replace the fuel filter about 15K ago, thinking about doing this again, although if there was something in the fuel lines, it should be intermittent as the blockage should move. This is constant.
- timing (re)adjusted
- tested and swapped out igniter. The secondary pulse resistance was high
- ECU and MAF swapped out with known good ones
- tested dizzy resistance
- dipstick shows beautiful oil, no white/cream color indicating a blown HG

I know the dizzy has a leak at the o-ring and dripping down the side of the head/block, but it's dry inside and should not affect the above. The issue is persisting after doing the above, so any help is welcomed.

 

[rolliges]

That's a lot of variables. Think simple things first; focusing on air/fuel delivery. That's all I got

 

[mingles]

If you haven't already checked, what is the condition of wiring harness near the EGR?

If it is not noticeable in Park but evidences itself when in gear possibly the movement of the engine is stressing an intermittent short in the wires.

 

[Malleus]

Any trouble codes? Normal or test mode?

 

[LINUS]

Is this a situation where it idles at a solid 650, but when you put a decent load on the motor it just starts falling flat?

Or is it fine through the RPM band sometimes & other times it's a dog?

Any chance it's something like a ground wire say between the block & body?

Have you been into the wires behind the glovebox yet?

 

[arcteryx]

Yes I was going brought the FSM following the order they specify based on symptoms, so all over.

No codes, I checked that too. EGR wiring looks good and I’d expect a code thrown if there was an issue with that, it has in the past.

I have only been in the glove of wiring to swap out ECUs.

At idle and in P, it idles fine. I can detect the miss very slightly sometimes when I’m opening the throttle manually under the hood. Once in the truck , it’s very noticeable outside of P and any load shows the problem regardless of RPM.

I’ll check grounds

 

[mingles]

Yes I was going brought the FSM following the order they specify based on symptoms, so all over.

No codes, I checked that too. EGR wiring looks good and I’d expect a code thrown if there was an issue with that, it has in the past.

I have only been in the glove of wiring to swap out ECUs.

At idle and in P, it idles fine. I can detect the miss very slightly sometimes when I’m opening the throttle manually under the hood. Once in the truck , it’s very noticeable outside of P and any load shows the problem regardless of RPM.

I’ll check grounds

FYI - At 48K and less than a year old my LX had a short in the wiring harness to the ECU where it passed through the firewall. Repeated stalling and eventually wouldn't run. It was towed to dealer and harness was fixed under warranty.

 

[LINUS]

That sounds like a plugged fuel filter.

Weird, since yours is fairly new.

 

[whitey45]

Pressure test the cooling system and pull the plugs, check for coolant in the combustion chamber, had one with the same symptoms and it was a small leak causing misfiring under load.

 

[Malleus]

Based on the information that the problem only occurs when not in Park/Neutral (I assume that when you "feel" the misfire under the hood you are seeing a separate failure; I could be wrong but it's easier to solve one problem than two), and based on the input/output ECM logic, my bet would be a faulty NSS circuit:

 

[Malleus]

FYI - At 48K and less than a year old my LX had a short in the wiring harness to the ECU where it passed through the firewall. Repeated stalling and eventually wouldn't run. It was towed to dealer and harness was fixed under warranty.

...or that.

 

[2fpower]

you said that : "fuel pressure was tested and passed by my indy." what does that mean?

I like to focus on parts of the 3 things needed. Sounds like ignition is covered, so fuel is my next thought. I had something like this before... step one we did the fuel filter and it fixed it. step two would have been Fuel pressure switch. Then injectors.

 

[gummycarbs]

My 94 was running very poorly shortly after startup until it was close to fully warmed up. It would idle okay reasonably well (it sounded like it was missing intermittently at the tail pipe), but when I tried to accelerate it would stumble and bog until I floored it, at which point it would rev out nicely. It was really weird.

I had already replaced all the vacuum hoses and cleared the passage of carbon that had leaked past a bad EGR modulator (make sure you do this, mine was absolutely packed with carbon!). I cleaned and reset the IAC and cleaned its passage into the intake. I tested fuel pressure at the injector rail (a PITA) and it was fine.

I replaced the fuel pump and, while I was in there, the check valve (you have to drop the tank a few inches and have a lot of patience and nimble fingers). I added a bottle of fuel system cleaner with PEA (the new hotness in injector cleaning).

Then I parked it for about a month and a half while I repainted the rig.

Afterwards, it ran great. Recently, the aftermarket fuel pump I put in failed. I put the original back in and it still runs great, so it wasn't the fuel pump.

So, the final solution to my weird stumbling and hesitation was one of:

* Fuel system cleaner with PEA and time to sit.

* Fuel tank check valve.

* Completely random event (something dislodged when changing the fuel pump, sunspots, gnomes, garage pixies)

No idea if any of that will help your situation.

 

[arcteryx]

Lots of good input. Some more history- I’ve owned this for 5 years , where it had say for 3.5 years. I put in a new tank, fuel pump, new fuel filter and redid the cooling system. About 3 years /7000 miles ago it was missing and determined by my indy (my independent mechanic) that it was a bad injector. I put in a new one and had the other 5 professionally cleaned. Ran great up until now. Point being is these systems are new, sure they could go bad.

I just chattted with my Indy and he suggested listening to the injectors with a stethoscope. I’ll have to pick one up as I don’t own one.

NSS. Hmm can you explain your thoughts on that?

 

[Malleus]

Look at the diagram I posted. The Park/Neutral switch is an ECM input. It's also the only one directly related to the transmission mode selection. I made the assumption (again, just for the sake of making the fault tree simple) that you have no problems with the transmission in Park or Neutral (there being no drivetrain load on the engine). If the ECM is getting a faulty signal, due to some circuit fault (switch, connector or wiring), it could interfere with the injector profile logic. (the ECM changes the injector profile based on the operating conditions)

Along the same lines, a faulty harness (through the firewall) would have the same effect. Based on your description, and the fault tree in the FSM, I'd start with faulty input to the ECM (I would start with the ECM, the FSM does, but you said you swapped it with a known good one), and work toward the fuel system from there.

Going along with your mechanic's advice, here's the FSM wisdom:

All of the other observations thus far are equally valid, given the fact that none of us are there.

 

[Trollhole]

Pull a vacuum on the EGR while warm and see if the idle drops and then release and see if it goes back to the same idle speed. You will need to cap your other side of the egr. Also take a timing light and run it under the hood and zip tie it to the windshield wiper. Do it for every plug while driving.

Harness
Engine compression hot
Radiator pressure test.

Could also be the knock sensor.

TPS checked out OK?

 

[BILT4ME]

Nobody mentioned it, but how about the intake hose? Has it developed a crack? Only under load.........

 

[ZackR]

When you say you checked the injector wiring by the EGR did you pull off the loom and get a good look at the wires themselves? On mine the loom/heat foil protection looked fine from the outside but when I cut it open three of the wires (nearest the EGR pipe) had been exposed and were shorting against each other.

I had about the same symptoms as you and it was very frustrating to troubleshoot. It would idle fine and then hesitate up until about 1900 rpm then smooth out a bit. Driveability sucked as you never knew exactly how hard or when it would misfire. No codes were thrown the entire time.

After I repaired the exposed wiring it was a night and day difference. Instantly smooth throttle response and no more misfires.

 

[White80]

Nobody mentioned it, but how about the intake hose? Has it developed a crack? Only under load.........

Was going to be my post, worth a look. Replaced mine recently after some stumbling. Found a 2" tear in it after snooping around, and much better now.

 

[inkpot]

Is the missfire constant or random? Constant implies always the same cylinder failing