1984 Chrysler Laser - idle hunting

My old Laser (2.2L w/Turbo I) just started giving me some strange issues in the last couple weeks. It bounces between 400 and 1,000 RPM on idle a few times, and then revs way up to 2,500 for a little while. Then it repeats the process. The vacuum gauge for the turbo reads up around 0 when the RPMs drop, and then backs off when the RPMs rev for the moon.

It was stalling, too, but I changed out the old plugs and cleaned some gunk off the distributor pickup which seems to have helped, but the idle problem remains.

Since this started, it has been giving me trouble on acceleration when cold, as well. It will stop accelerating, and then buck and surge until I either let up off the gas, or “push through” causing it to take off like a rocket.

I did a basic check for vacuum leaks (I don’t have a gauge) by just checking for loose hoses, and seeing if I could cause anything by bending or pinching the hoses, but I didn’t notice anything definitive.

(on a completely unrelated note, I just realized I misspelled my username)

Does your car utilize a throttle position sensor? If it is failing, it can cause all of these symptoms.

Yeah, it uses a TPS.

I unbolted the sensor (I couldn’t pull the contacts out of the plug head with the tools I had handy, and it shares the plug head with a few other sensors, so I couldn’t just unplug it) to see if that would have effect. Bizarrely, it didn’t seem to change much, which doesn’t seem quite right. I hooked it back up, and then the idle wouldn’t drop below 1,900.

I actually have a TPS on the way, because I found various parts on wholesale and I figured it wouldn’t be bad to have some spares (even if they are cheap, they’ll do as patch parts). I will pop it on when it shows up sometime next week.

I hope this turns out to be your problem. More recent vehicles are more resilient to any one sensor being bad or intermittent, but early computer-controlled vehicles really flake out with a bad sensor. I had an 85 Ford that had very similar antics to what your Laser was doing, and it was maddening until I replaced the TPS.

Well, I replaced the TPS. No change.

To date I have replaced the ignition module, cap, rotor, plugs, wires, and TPS. Everything related to the ignition needed to be replaced sooner or later anyway, it was all original equipment, and was worn pretty good.

It seems more prone to just dying on idle now. It will bounce and hunt for a little bit, but won’t “snap back” as well, and just dies completely. Now sometimes it won’t start up again unless you put your foot on the gas.

Now to me, that would suggest a fuel draw issue, but the pump itself seems strong under acceleration, so that would strike me as unlikely. Could be the MAP, I suppose. Those were known to go bad on the Laser / Daytona.

I suppose it could be the MAP, but if it accelerates OK, it probably isn’t.

Yeah, that’s why I didn’t want to replace it unless I had been through everything else first.

Here’s an idea, though. What about the Air Charge sensor? It was a thermistor + resistor on the 84’s, which would follow with some of the symptoms. Namely that all the symptoms go away as long as you keep the engine temp up; if you let it cool down, the idle will fall apart again. If you keep it warm, it will idle right at 950 and accelerate like a charm. If the thermistor was bad, it might not be getting a proper reading at low temps, so it could throw off the idle.

Not sure how that would explain the hunting behavior, but that might be other systems trying to compensate for the engine knock that occurs when the RPMs drop super low.

When these cars were 5-7 years old I often found the problem to be in the brittle injector wiring harness. Wiggling the wiring harrness near the injectors would cause erratic behavior (engine speed flare). The computer ground goes through that harness and is bolted to the manifold. That harness becomes brittle from the heat of the exhaust manifold. Just a wag.

I reset the ECU back into learning mode, and it seems to be running smooth now. Idle is still a little high (normal high, not crazy, bouncing high), but it’s still tweaking things (I’ve driven less than 15 miles since I reset it) and seems to be edging down closer to normal again.

I think the ignition module was flaking, which is why it would develop problems going from open to closed loop (controller for that was on the ignition module for that model), and I just needed to reboot the system to make it learn to play nice with the new parts I put in.