Intermittent miss problem. Any ideas?

[fast_eddie_72]

Seems like there are some really smart and helpful folks here, so thought I'd ask. This is one of those tricky ones and probably very hard to diagnose on the internet. But thought it worth asking in case there are some common issues I haven't found yet.

The car is a 1991 Stealth RT/TT. Totally stock when I got it. It ran great for several months. One day on a long trip, we stopped for lunch, got back in the car to take off and had a severe miss on the way out to the highway. It cleared up as quickly as it started. We didn't think much about it and got on our way.

Then it started to happen more often. It seemed to be worse on really hot days. A bad miss under any kind of load. If you go really slow and up shift at less than 3000 RPM, you can drive the car, but it's no fun and a real pain. Probably kind of dangerous too. It seemed to become a "more often than not" kind of deal in the hot July days with temps in the 90s. After reading about ECU issues, I figured it couldn't hurt and would need to be addressed anyway. So I bought the Jester Chrome ECU. I have some fiddling to do with it still (subject of a different thread) but it works and the car runs. We thought it solved the problem, and it does seem like it's happening less often, but that doesn't make sense to me. It's either the ECU or it's not, and it's clearly not. Maybe it's just luck of the draw on an intermittent problem.

When it's running right, it seems 100%. Super fast, pulls really strong. When it's acting up, occasionally it's relatively minor, but usually it's pretty bad. Like "I hope I can get this heap home" bad, at least in town. On the highway, once you get up to speed, it's pretty okay. If it's missing, you don't notice it.

I just ordered plugs, wires and gaskets for that job. I figure that's the place to start. That might fix it, but somehow I doubt it. We'll see. I kinda thought it could be a failing fuel pump, but I'd have expected it to fail completely by now. If pugs and wires don't do it, I'm kind of thinking maybe a coil pack going out? I don't know. Seems like it's worse than that. Even with one or two cylinders not firing it would accelerate some. I'm planning to pull and test them when I have the car apart, but not confidant that is the issue. It's pretty bad - kind of jerks the car. That feels like a lean misfire somehow. But Who knows?

Intermittent problems really suck. It sure does seem to be heat related. On a cool morning, you start the car and it runs fine for a good while. Middle of a hot day with the car outside, it's hit and miss. If the car is warmed up and you stop for a few minutes, like to fill up with gas, there's a good chance you're going to have trouble. A little highway cruising and sometimes it clears up again. That all makes me feel like it's something electrical, but it's just a guess.

Thanks for reading all that and thanks in advance for any help.

Ed

[CoreyB]

The plugs and wires would be a great start. A bad plug wire or 2 could cause this. Also you may want to check all the injector wiring, on cars of your age the wires become brittle.

Kinda surprised you are not getting a CEL, if you did that would be very helpful. There are so many things that could cause this that it can be very hard to tell without a CEL. Does the light work?

[fast_eddie_72]

Wellllll - I am getting a CEL, but I believe it's because the peripheries aren't set up properly on the Chrome ECU. I have't quite figured that out yet.

We did get a CEL with the stock ECU. I guess it would be a good idea to put the stock ECU back in and check the codes. I was never really clear how to do that but I know the information is out there.

Shoot, now that I put that in writing, I absolutely need to do that. I was hoping the ECU would take care of the problem, but since it hasn't, it's just another variable I've introduced. At least with the stock ECU everything is 100% stock.

[fast_eddie_72]

Can you use a light rather than an analog volt meter to read the codes? I don't have an analog volt meter, but I'll go find one if I need to.

[CoreyB]

You might be able to use a light, some test lights will work but some might not. Its only a 7 volt pulse. Stealth 316 - Reading Diagnostic Codes - 1991-1993 3S models

[fast_eddie_72]

I'll just grab a meter. Thanks for the help so far! Sometimes you just have to talk something through to find the obvious answer.

[fast_eddie_72]

Okay, I think I got this.

I put the old ECU back in. Drove it around for a while. Got the symptoms and FINALLY got a CEL. Hooked up the meter and pulled the code. One long, four short. TPS.

Here's the thing. When I had the Chrome ECU in, and checked the code it was sending, it was also for TPS.

So can a throttle position sensor cause the symptoms I'm describing? I'm going to have to figure it can somehow, because that seems like a positive result. I just went through a bad TPS on another car and it didn't do anything like what this is doing, but that's the only bad one I've ever had, so I don't know what they might do.

Hoping someone has had similar experience, but I'm going to start looking at TPS options on Rock Auto. Really sounds like that might be the problem.

[CoreyB]

Yeah a TPS can do what you described. Glad you were able to get a code. Post back after you change it so I know if that was the problem.

[fast_eddie_72]

I will! And thanks for the help. I feel a little silly for not just reading the code in the first place. It's looking like that's the problem and I sure did make it harder than it needed to be. Talking it out here really did help me see the obvious answer. lol Amazing I can fiddle with cars all these years and still miss the simple answer.

[fast_eddie_72]

A quick follow up here, because this may be very helpful info to someone. A friend who's a professional mechanic advised me to clean the connector to the TPS. He thought the intermittent nature of the problem may indicate that it was just a dirty contact. I did it Friday night. I didn't really think it would work, because everything looks great under the hood. The connector looked clean, but I sprayed it with some contact cleaner and worked the plug off and on a few times. I drove it quite a bit yesterday and today and have not had the symptoms or a CEL. I'm hesitant to call it fixed. You never know with intermittent issues. Sometimes it's just a lucky day or two. But I've tried over and over to recreate the conditions that would cause the problem. When I put the stock ECU back in to verify the code, it only took me 20 minutes to get it to act up. So I'm mildly hopeful that this is taken care of. I put a meter on the sensor and worked the throttle and the response was very smooth. I didn't see any dead spots at all. That analog meter I bought to read the code was really good for checking it.

Anyhow, I just went out and put a dab of dielectric grease on it and put the little wire clip back on the connector. Took it for a short tromp up a couple of entrance ramps just for the grins. It didn't miss a beat. I'm cautiously hopeful this one is put to bed. Of course I already ordered the part. But if the issue hasn't shown up again, I'll probably put it in a box for down the road. TPSs do fail eventually and we plan to have the car for quite a while.

I'll report back either way in a week or two. If this is the solution, good to have it documented somewhere.