I've been chasing a miss for some time. The engine runs a little less than smooth at idle. It stumbles under load. The problem occurs in both open and closed loop. I could use some suggestions.

The problem came on suddenly after a long trip when the gas tank was getting low. I assumed some crap got sucked up from the tank and clogged the filter. I replaced the filter and verified the injectors were clean and firing correctly. No change.

I've gone through the ignition. I ohmed the wires and verified no arcing. The distributor cap, wires and rotor are nearly new. I replaced the plugs. No change.

I have no codes.

 

Take a look down the board list for ahaaa. His was simply a loose connection at the fuel pump. Could be?????
Chuck

 

That's a good thought. I failed to mention that as part of the troubleshooting I pulled the pump and replaced the pulsator with fuel line hose. Fuel pressure is steady.

 

Could be a bad injector(s).

 

I thought so too. I installed a fresh set from FIC.

 

I've seen (on my Own '85) a faulty ignition module (in the dist.) cause sort of those symptoms. Stumble/miss/shake at idle, almost like the mixture is way off. Occasional random miss under Some load conditions.

No codes, not even tho the OBD1 is Supposed to be capable of seeing primary ign. problems. Not the fuel, playing with the chip (pulse width vs. load) can make it Worse, but not better. Scan tool values all look fine, so does checking spark with a timing light. A Scope might have shown something happening, I don't know.

For what it's worth I've Also seen dying plug wires do nearly the same thing. But you have checked that..

 

Thank you for the feedback. I've been wondering about the module but according to the FSM any problem with it would throw a code. Your symptoms are very close to mine. Engine has an unexplanable subtle shutter at idle. Random misses under load. My scan is fine, timing looks good, etc. I have a spare module. Looks like its time to drop it in. Thanks again!

 

I just tried installing a spare module that I know is good. Still no joy. I appreciate the response. This is one more thing I can scratch off the list of possible causes.

 

Hmmm. So much for That idea.

I went round and round with mine, chasing vacuum (looking for leaks, etc.), verified the compression check/valve adjustment, was about ready to pull the front cover off and look hard at the timing chain - if the timing had been Doing anything it shouldn't.

Verified fuel pressure, swapped out injectors, new O2 sensor, etc. Drained all of the gas out of it and started fresh. Plugged in the prom emulator and started playing with the fuel table - I could make it Worse (going leaner), but not better. Nothing helped.

I removed and/or plugged All of the vacuum lines (all of the emissions stuff is long gone Anyhow - no AIR or EGR). No help.

Then began the game of throwing parts at it, although since I have a second vehicle (older truck, long story) with an identical '85 TPI setup in it I can just swap parts back and forth until the problem moves to the other vehicle.

That happened when I swapped the distributor, and a different module then cured the issue in the truck, once I got it there.

I had never seen a module do anything in the way of going bad other than just flat Die. That was a first. And no codes, so much for OBD1 being aware of whatever the module was doing that was making the idle behave strange, and I Still don't know what that might have been - the Timing looked Fine, using a light.

Like I said tho - plug wires cross-talking (or a really bad cap..) can cause the same sort of surging/unsteady/weird idle problem. For that matter so can water in the gas, vacuum leaks, issues with the MAF, bad coolant or intake air sensors, etc. A scan tool with all of the numbers looking Normal, however, can drive you nuts.

All of the Input numbers, and the assocuated Output number - the Base pulse width. No way to tell on the '85 what the commanded timing advance is, but you can see that with a light.

Did I mention I had also swapped out the ECM? Right along with the MAF module, right After the MAF itself and the ESC module.

The ECM Will detect a faulty pickup coil in the distributor (that was a different learning experience..). It will NOT see anything happening in the secondary side of the ignition, as in the Coil itself, or the wires.

Here's a thought - how good is the distributor wire harness connector? Is That connection "clean", no high resistance connections? Again, something that if there was a problem you would Expect the ECM to see it and set a code, but..

I guess I can offer this - do you still have a functioning EGR valve? Checked That to make sure it's really sealing closed all the way? Vacuum leak somewhere else, like the Brake booster? or the charcoal cannister?

Here's another thought - does the idle issue change at all if it's under any Load, like in gear (if automatic), or the A/C on/off? Is the IAC able to to regulate the idle rpm with reasonable accuracy?

Doesn't explain why it would stumble/miss under a load while Driving, tho, does it. Got to be either a fuel problem (going way too lean, most likely) or a spark/timing thing.

I feel your pain - when you Do find it (and it's a finite list of things it can be, afterall), I'd like to know what it was. Just for curiosities sake

 

Wow! I feel some comfort knowing someone else has gone through this. You bring up some good points. I have the old distributor and coil. I could swap out the whole ignition system from the primary wire to the spark plugs. It won't cost anything to try.