I dug out the old Bird at the beginning of the month. It started up fine and ran OK so I took it for a cruise.

Once it got warm it started to stumble at idle. It's rough in neutral and even worse in gear. It's not bad enough to stall out but the shake is annoying. It accelerates OK and runs fine up at 1500+ RPM

I did a quick tune-up (plugs, distributor cap and rotor). It runs fine when first started cold, but it still stumbles when it gets warm.

I can dig into it further, but I wonder if anyone has 'the answer' on top of their head I haven't checked the plug wires yet, but I'm thinking if there was a bad wire, the miss would be there when the engine is cold too.

No trouble light or codes.

I've had the car for 25 years... In the past plugs and a distributor cap always fixed any rough running conditions. It's got 100K on the clock.

 

Maybe bad gas if its been sitting a while.

 

Check your injector ohms cold then again once warmed up. You might have 1 or 2 on there way out and this will cause your problem. Also if your icm is starting to ho out as well. Remove it and have it tested

 

X2 on the injectors.

 

Definitely check the injectors, but also consider they may be sticking and not opening the entire way causing fuel starvation. Do you have a scanner or logger to see the 02 readings?

Could be as simple as gas, could just need an injector service, could require replacements. You could try a tank of Premium fuel with their "detergents" and a bottle of high quality fuel system cleaner see if maybe that corrects it, but not being able to get over 1500rpms will make that difficult.

Are you sure you are maintaining PSI on the fuel rail? Has the Fuel Pressure regulator diaphragm rotted and not controlling pressure? Has your fuel pump gotten weak, clogged fuel filter?

Few things to check, hope this helps.

 

Thanks for the replies guys... Sorry I didn't get back to this thread sooner. I had some home renovations to get done, and the diagnosis had to wait.

I got back at it today and actually made some progress.

It seems that it isn't a random miss. The idle is great when started cold, but it begins to run rough and surge as the temp gauge needle starts to come off the cold mark. The surge was so bad that the car would jerk forward at red lights if I didn't hold the brake down hard or put it in neutral.

So, I checked for a vacuum leak and couldn't find one. Next I went to the IAC and cleaned it. It wasn't that bad, not much carbon. Cleaning it made no difference.

Next I looked at the TPS sensor. I re-seated the connector, sprayed the side of the TPS closest to the throttle body with some electronic cleaner, then I worked the throttle back and forth a few times.

That made a big difference!! The idle still isn't perfect when hot, but the bad surging is gone. Even at full operating temperature, I don't have to pop it into neutral any more at red lights.

I'm going to pick up a new TPS and replace it. (Do they even sell these things any more)??

Thanks again for the help...

 

 

Another vote for checking your injector ohms cold and hot.
My injectors Ohmd fine cold(and the car ran great cold), but once warmed up it turned out that I had four injectors that ohmd way out of spec.