Stumble on all RPMs

Xanth · Post by **Xanth** » Fri Jul 13, 2007 7:54 pm

I've been driving this thing for several months now with an annoying stumble that is really getting frustrating.

What I've replaced/done:
Dizzy Cap and Rotor
Ignition Module
Coil
Spark Plugs (ACDelco's)
Wires
Fuel Injectors and Rail
Cleaned Throttle Body
Blocked off EGR, no difference

Whats still happening:
The engine has a stumble across all RPMs, but still has plenty of power. It idles nice and low, any sort of vacuum leak would make it idle high correct?

Could I perhaps find a vacuum leak by comparing a 'Idle RPM vs. IAC' WinALDL log from my car vs a good running car?

With the engine off I get ~42 Psi, though it does visibly drop once the pump has shut down, less so since replacing the injectors. I would say it dropped down to 30psi after 10 minutes or so, it didn't go below 30 while I was watching it. I replaced the injectors with a spare set because the ones I had leaked, these don't seem to as I no longer have a hard warm start or fuel smell on a warm startup. Is that pressure drop too quick? I have an eBay fuel pump right now with what may be the original pulsator. Either one of these could be a failure point.

The engine itself now has 98k miles, it sat for about 7 years before I got it and put it on the road. It seems mechanically sound with no abnormal noises and no apparent burning of oil.

Any recommendations? This is really frustrating me, each part I replace makes a positive difference as they are all old, but nothing is fixing this issue.

whipped · Post by **whipped** » Fri Jul 13, 2007 9:49 pm

Take a look at the TPS readings as you slowly and smoothly open the throttle. Check for any sudden jumps.

What's your MAP readings at engine off, and idle?

p8ntman442 · Post by **p8ntman442** » Sat Jul 14, 2007 6:27 am

Check fuel injector harness grounds. Could be intermittent.

Xanth · Post by **Xanth** » Sat Jul 14, 2007 6:06 pm

From an older log I have but still having the problem, the MAP reads 0 off and fluctuates all around 40 at idle. On the aldlview graph it seems to fluctuate in both directions by 3.

Forgot to mention the TPS is also brand new, replacing it didn't have any effect on my problem.

I'll check my harness grounds tomorrow as I'll be doing a lot more diagnoses then.

What I'm describing as a stumble is really more like hiccups, however according to my Tach each hiccup actually results in momentary increased engine speed.

It will run smooth for a second or two then hiccup I believe 3 times, then go smooth again. This is the same across all RPM's at all temperatures. It tends to be fairly rhythmic, however it does not occur during every firing cycle, perhaps once every few cycles. It seems to pulse in a series of 3. Smooth for a moment, then 3 quick hiccups combined with slight RPM jump. It runs smooth longer than it hiccups, if that makes sense. It has been doing this constantly since I put it on the road a month or two ago, nothing I have done is resolving it and it has never not occurred.

The idle however seems better when I am in diagnostic mode, not sure if the hiccup is just less visible or actually gone. It still remains when driving in Diagnostic mode.

Tomorrow I will do the checks and get a new ALDL log.

Shaun41178(2) · Post by **Shaun41178(2)** » Sat Jul 14, 2007 6:10 pm

plug wires?

Xanth · Post by **Xanth** » Sat Jul 14, 2007 6:27 pm

Brand new, changed twice now to no effect.

Dirty Sanchez · Post by **Dirty Sanchez** » Sun Jul 15, 2007 8:27 pm

Have you checked your pickup coil?

Xanth · Post by **Xanth** » Sun Jul 15, 2007 9:22 pm

Dirty Sanchez wrote:Have you checked your pickup coil?

I have seen it, but not done any checks on it. It is not pretty, there is a layer of wrap or something on the outside of it that seems to be disintegrating. Could this part failing cause a problem like this? Any way I can test it?

Edit: I see Fiero store lists it for just $7, I'll just get a new one.

Here's the checks I was able to do today, not as much as I hoped:

Vacuum at idle reads 17, changes directly with the idle of the engine, rises as I rev it. It jumps only as the engine is having its hiccups.

I tried disconnecting and plugging various vacuum lines, was unable to gain any improvements there.

Fuel pressure is about ~40 engine off, I can watch the pressure drop. When I first installed the pump it was at ~45 PSi engine off.

Idle Fuel pressure is down around 30-35 psi.

Distributor has no play side to side, and very little rotational play.

I got a negative Voltage reading when connecting the + of my volt meter to the ignition module screw and the other end to a good ground.

I'm suspecting a problem with my fuel pump or pulsator, I don't believe I should be able to watch the fuel pressure drop down so quick. The car no longer shows signs of leaky injectors since I replaced them, hard start is gone as is fuel smell on startup.

lucky · Post by **lucky** » Thu Jul 19, 2007 6:47 pm

To test the pickup coil, pull the coil plug off the ignition module and use an ohm meter on the terminals. Resistance in the coil should be almost Zero.
If there's no resistance test with a continuity tester, and play with the wires while you do to verify that there is no short.

The Dark Side of Will · Fri Jul 20, 2007 10:01 am

pickup coil resistance should be between 500 and 1500 ohms. Neither pin should show any connection to ground.

Xanth · Post by **Xanth** » Sat Aug 04, 2007 8:25 pm

Just to add closure to this issue, after throwing tons of time and money at this thing, turns out it was actually the ECM at fault.

Not sure if it was defective when I got the car, or if I somehow managed to fry it. I got a replacement from Chris Hodson only half-hoping it would make a difference and what do you know, the car runs mint now.

After replacing damn near every likely part, turned out to be might-as-well give it a try one. I'm just happy I can finally drive the car like normal and enjoy it, it was just frustrating the crap out of me before.

The Dark Side of Will · Sat Aug 04, 2007 10:20 pm

Interesting... I'll have to check out the GM trouble shooting tree to see if they include a "replace ECM" ending.

I just found out that $22 plug wires have a 6 month life span. I replaced them (they've been on a year) and my ignition problems went away.

Xanth · Post by **Xanth** » Sun Aug 05, 2007 12:09 am

There didn't seem to be any indications that the ECM itself was the problem, ALDL data wasn't horribly off, it just clearly showed my engine running like crap. Also no error codes, but I would get proper error codes with sensors disconnected.

It was also a reman unit, so as far as I was concerned I had a new computer. Not sure if this is possible but its almost as if it wasn't properly firing one bank of injectors, when I paid real close attention to the stumble it was in a series of 3.

I may open the thing up just for the hell of it, just wondering if there's any blatant failures on the board.