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Sometimes missing engine

I have a 1991 Ford Explorer with about 103,000 miles on it. Sometimes and only sometimes when it’s a cold startup it’ll idle very rough and sound like it’s missing a cylinder or something. It’s a 4.0 V6, and when it acts like this it’s hard to get onto the freeway as in it seems to be lacking power. Any ideas what’s going on? Thanks Elliott

Trying to think… sounds like a sensor. IROC MAN 84. Thats funny. I have an 84 IROC-Z. Fast car.

Have you changed the spark plugs and wires?

Yeah I had an 86 IROC, loved it but couldn’t keep up on the repairs. Yes I was thinking sensors too since it’s not an all the time occurrence.

Yes the plugs, wires and coil, have about 10,000 miles on them, so they should be good still.

What about after it gets warm? Does it smooth out at all with some throttle? In addition to reporting on the condition of plugs & wires, also note other maintenance items (e.g. fuel filter), and check your coolant temp sensor.

There could be several problems. If the engine only runs ragged when first started cold this can be caused by a loss of residual fuel pressure. (generally a leaking check valve in the fuel pump)
If it straightens out, you shut it off, and then start it right back up with no problem then that would point even more to a FP check valve.

The lacking power onto the freeway could be a fuel pump getting weak and/or a clogged fuel filter. The latter can kill the former if neglected long enough.
Remove the filter, let it dry for an hour, and then blow through it. You should not feel any restriction.

Another possibility is a failing ignition module seeing as how you have one of the infamous TFI models. Do a net search about “TFI Settlement” and that will provide the details behind these modules.

Thank you for the thorough answer, I can say it’s not the fuel filter, I thought it may be clogged and replaced that but with no effect. I’ll have to look into the fuel pump and look up TFI problems.

Yes it smooths out eventually, sometimes between 30 minutes and sometimes 30 seconds. It doesn’t really smooth out with throttle through. How would you go about checking the coolant temp sensor and how would that effect the way it’s running?

The computer uses the coolant temp sensor to figure out the air/fuel mix. On older engines with carburetors there was a “choke” - this cut air to make the richer mixture needed to get the car running when it is cool. On fuel injected engines the computer figures out how or if to “choke” it (by making a richer/leaner mixture). It uses the CTS to do this. If your CTS isn’t letting the computer know the engine is cold then you are running too lean a mixture on start up.

To check it you need a digital multi-meter and the resistance specs for the CTS. There is a baseline resistance to the sensor when cold and that increases as it heats up. That’s how the computer gets its info. Meters can be had cheap if you don’t have one. You can probably get the specs from Autozone’s online repair manuals (free). You can check these on the engine or pull it off and stick it in a pot of water that you can heat up.

Excellent, that sounds like it could the problem or at least a future problem, I wonder if it wouldn’t be best to just replace it. Thank you for the answer.

Thats true. Autozone sells a fuel pressure guage for around 30 bucks. Check your fuel pressure and see that it is where its supposed to be as per OEM specs. Also The TFI modules, Thick Film Ignition modules, are notorious for going out.

Will do, thanks.