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'89 odd engine misbehavings

Aug 30, 2002
Colorado Springs, CO USA
84 Z51 auto R.I.P. 89 black roadster SOLD
This all started driving home a few weeks ago...went to take off from a red light and the engine started "surging" and eventually died. Would not start back up. I had been wanting to address the exhaust system anyway, and in doing so found the main cat was full of pieces from a disintegrated pre-cat. So addressed that, all three cats are clear and installed new rear y-pipe, new mufflers and O2 sensor.

However this problem still exists (which is not terribly surprising, I didn't really think it was an exhaust restriction problem, but it was something I wanted to do anyway).

Car starts up and runs/drives GREAT when cold (and the sound is downright intoxicating!). I can drive it around the block for 10 minutes or so (during which it runs like a wild beast!), park it in the garage, shut it off and it'll start right back up and run fine. Let it sit half an hour to an hour or so, and it starts acting up again like so:


This behavior is very repeatable, and there are no codes stored.

Fuel pressure is good during this behavior, it does not fluctuate at all.

Manipulating the throttle does absolutely nothing when it is doing this weird surging thing.

Per FSM (and recommendations by several experienced friends), disconnecting the MAF when it's acting up has no effect...the problem is still there. Also replaced both MAF and burn-off relay...just because. I can feel both relays activating appropriately (MAF relay while running, burn-off relay for about one second shortly after shutting the engine off).

There is another mention in the FSM that says "If engine starts but then immediately stalls open distributor, set timing connector. If engine then starts and runs OK, replace pickup coil." I honestly don't know what the heck this is saying to do? The "timing connector" is not inside the distributor. ;shrug

Unplugging the EGR vacuum line also has no effect.

I wanted to try disconnecting the IAC and see what that does. In doing so I inadvertently forgot to re-connect the TPS (was disconnected to provide enough room to get to the IAC connector, just forgot to plug it back in), lo and behold it started right up with no surging whatsoever (of course with a bit higher idle than normal). Noticed the TPS had been left unconnected, plugged that back in, still started up just fine. Re-connected the IAC, and again started back up and idled perfectly.

It does "sound" like a confused IAC that just can't figure out what to do...but shouldn't the thing still idle with a completely closed IAC? (just very low). Even if the IAC is "confused" or whatever...giving it some throttle should eliminate this surging if it were due to the IAC misbehaving.

I haven't pulled the IAC yet to look inside there...but I would think any restrictions there would be present ALL the time, not just after a warmup/partial cool-down cycle.

Haven't measured TPS voltage yet either...will do that next. However, that should set a code if it's not right.

Kinda stumped...

EDIT: Problem found. 5.6 ohms at #5 injector. D'oh!
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