RKeats

Fellers, help save a GTA’s life by preventing me from burning it to the ground. Car’s a 1989 with its original L98 / Auto. Bought it and it was running rough right from the start. Catalytics were clogged - I replaced both. No improvement. Found an injector that was close to death so I replaced them all with Bosch 22lb’s from Southbay, no improvement. Fuel pressure shoots to 50ish, then holds steady around 40, and all injector resistance values are floating around 18ohms. Plug wires were crusty - replaced. No vacuum leaks to be found. IAC, TPS, CTS, MAF, all seem to be working fine.

Car will sometimes start instantly, sometimes it’s tougher, but it always gallops when cranking as if it’s down a cylinder. When it starts, some times the idle wanders, sometimes it doesn’t, but it always shakes a typical misfire shake. No codes being thrown or stored. Timing has yet to be checked as I don’t have a light, but wouldn’t it have to be WAY off to cause such a sassy miss? I’m not a certified mechanic, just a learner. I also can’t seem to find anyone else with an identical issue, or the few forums that are close enough just end with the O/P disappearing.

As an extra test, I’ve tried running the car with the IAC/MAF/TPS unplugged (separately) and each time it runs differently, but once the sensor’s plugged back in, it doesn’t return to it’s previous state. Is that because the ECM needs time to relearn, or is that hinting at the issue?

help

Komet

Gallops when cranking is suspicious. I'd do a compression test, maybe pop the valve covers and have a look at how the rocker arms are doing, watch / listen to them for weirdness.

RKeats

I was actually just out there again and decided to try running it with the EST unplugged, as I’m under the impression that it’ll override sensors and the ECM’s timing adjustment. Car ran MUCH better, idled, and seemed to lose the misfire shake. Drop the rpm’s in drive and it ran poorly again, but more like it was about to chug and shut off vs a consistent shake. This would point me back in the direction of checking sensors, no? It sounded great when it was running. No knocks, no valvetrain noise, not smoking, nothing notable at all really besides the inability to idle low.

tajefe

I'm trying to solve this almost exact issue and would prefer not to keep tossing parts at it. Did you or anyone ever solve this?

hessm70

So recently I had a similar issue ('88 GTA L98), I had replaced the valve seals and decided to replace the distributor cap. Car would start but ran like crap. Replaced old cap then still had an issue. I took an endoscope into the spark plug holes to see if there was anything or observation of damage in the cylinders. Put plugs back in and installed the wires. Car started and seem to run fine. I did double check the plug gaps but nothing was crazy off there. The only thing I can figure out is it was a wire not totally seated on the plug.

I agree with compression check and check distributor cap, rotor, plugs and wires.