Engine rough misfire on hot restart

The car is a 1991 Chrysler LeBaron Convertible, with the 3.0L V6. The car has “only” 162,000 miles on it, but the engine is pretty worn out - we’re running 5W50 synthetic oil to keep the smoking down and the oil pressure up.

Up until a couple of months ago, it ran decently smooth, just down on power. Now however it runs very rough sometimes, with very little power. I suspect all of the usual things (plugs, wires, distributor, fuel filter, etc.), which are all about 3 years old, except for the fuel filter, which came with the car when we bought it 4 years ago. But here’s the thing, it only runs rough if it’s restarted after running. If it’s allowed to cool for 4-plus hours, it usually starts up ok. If it starts rough, it stays rough running for the trip unless it gets a good workout on the highway, then it MIGHT clear up the misfire. If it starts fine, it typically stays fine.

The Check Engine Light usually comes on when it runs rough. The codes I get from making the CEL blink are 12 and 26.

Haynes says:

12: battery disconnected recently.

26: something about high resistance in one of the injector banks.

So, where to start? Could the coil be going? Some capacitor somewhere? Just worn plugs, etc? Any help is appreciated.

The rough running is variable - sometimes it’s a little rough, so it struggles a little bit on the hills, and sometimes it’s very rough and it’s hard to get up to speed.

That makes sense. The resistance in the injector bank means one or more of your injectors is not fully opening. Most injectors just short out and don’t open at all. You may have a combination of shorted/high resistance injectors. This will definitely cause a misfire. A blinking CEL means something major is wrong with the engine. Have a good independent mechanic check it out.


FYI, the CEL doesn’t blink while running, it just stays steady. The blinking I’m talking about is the diagnostic system - you turn the key on off on off on, and then the CEL starts blinking the codes at you.

I’m reluctant to assume its the injectors right now, despite the code. We had a problem a few years ago with rough running, although the symptoms were a little different. The injector codes came up then too, but nobody could find anything wrong with them - in the end, the computer was replaced, which fixed the problem.

Am I wrong in thinking that heat soak of some components can cause this?

The diagnostic blink is normal. The ECM controls the injectors so a bad computer would affect them. A heat soak is certainly possible with modern electronics so now all you have to do is find out what’s being affected. The coil or ignition control module are likely suspects to get you started.

Well, hopefully it’s the coil, because the ignition control module is gonna cost me 300 bucks! Plus I’d have to figure out where it is…

So how do I check this out? The internet gives me various ways to check the coil with an ohmmeter. In this case I guess I should compare hot and cold readings?

If it’s the module, is there a way to know without just buying a new part?

those car had a big problem with the injectors. so don’t be to fast to count them out.

Is there an easy way to check them?

you can ohm check them and see if they are with in spec, also hook up a fuel pressure tester and start car and then turn off the pressure should not drop at all. if the pressure drop you have a leaky fuel injector.

So here I am almost two weeks later.

I haven’t been able to check the injectors yet per Hemi’s suggestions - I can’t seem to find any specs on injector resistance to compare to, and I haven’t gone out to get a fuel pressure tester.

I DID do the easy stuff - changed plugs, wires, distributor cap and rotor, fuel and air filters, PCV valve (they were all due anyways). No change to the problem. Five of the plugs came out looking pretty clean, but one had a large amount of soot around the electrode. Attached is a picture of the dirty plug and a typical clean plug.

Is it safe to assume that it’s a faulty fuel injector? How about the wiring? Once I open up the plenum to change whatever needs changing I’d really like to be able to get it in one shot.

My opinion is that both of those plugs look pretty bad, with one much worse than the other.

I’ve always felt that whenever an engine performance problem exists (and especially so on a high mileage, self-described worn out engine) that a compression test should be run when the plugs are out. If a cylinder is down on that last plug any repairs will only be a stopgap measure until the engine is fixed.

If an injector fault is suspected you might check the resistance with an ohmmeter and my memory is very very fuzzy on this but look for around 15 ohms. You should get the same reading on every injector.
With the engine idling, try listening to the injectors with a stethoscope or long handled screwdriver and make sure each one is faintly clicking away.

Well, I finally got out to pick up a fuel pressure tester and compression tester today - should have the results tomorrow. One thing is driving me bananas though - it seems that Chrysler/Mitsubishi neglected to put a Schrader test valve on the fuel system, which means putting a tee in the fuel hose. Can’t find it on the car, in the manuals, or on AutoZone. Could somebody confirm for me that Chrysler didn’t simply hide it behind a fender, under the doohicky beside the whatsit?

Here’s some results:

Compression test, cold engine, 1st run:
1: 180 psi 2: 185 psi
3: 167 psi 4: 185 psi
5: 180 psi 6: 185 psi

Both Haynes and shop manual say that 178 psi is spec. I’m going to check with Chrysler tomorrow to confirm, because when I squirted oil into each cylinder and tested again, I got way higher numbers:

1: 225 psi 2: 235 psi
3: 220 psi 4: 240 psi
5: 235 psi 6: 235 psi

I took the air plenum off. There’s a thin coat of oily dirt all over the inside of the plenum and crossover tubes. Is that normal? Also, the gasket was torn at the #5 crossover tube. Not sure what that was all about - the tube wasn’t any dirtier than the others.

I put the two-week-old spark plugs back in. Back three (1-3-5) were pretty clean. Front three (2-4-6) had dirt already building up on the exposed metal surfaces. #4 was especially dirty.

Checked the injector resistances. All were 2.6 ohms, except for #2, which was 5.2. But it was #4 that looked like the biggest problem. Go figure.

Fuel pressure test gave me 47 psi, which is pretty close to spec I think.

So, where to go from here? How do I get rid of the misfire?

I think late night posting is working against me here…

Well, it appears that the #2 injector was indeed the culprit. It had a higher resistance than the others, and the crud in the injector socket was wet with fuel while all the others were just dry crud. It took three tries from the parts houses around here to find a working injector for a car this old, but it seems to be working now.

All this work leads me to a 2nd question - the electrical diagram from AutoZone shows only 4 wires for the six injectors - a common, plus 3 control wires from the computer, that each split to control two injectors. Doesn’t this mean that each injector is firing once on the intake, and then again at a wrong point in the cycle?

Doesn’t this mean that each injector is firing once on the intake, and then again at a wrong point in the cycle?

Yes, that’s probably exactly what that means. Fuel injection systems when they first came out were kind of primitive compared to the ones we have now. So I’d say that this one is somewhere between a throttle-body injection and a true sequential injection system in efficiency.

Also, you can probably safely disregard the code “12” you’re getting. A lot of Mopars of this era threw this code for no apparent reason. I think the older computers didn’t deal well with the voltage drop when you started the car, so they mistakenly thought the battery had been disconnected. If your battery is good and your connections are solid, I wouldn’t worry about it.

As unbelieveable as it sounds, check the camshafts. I found a broken camshaft causing that problem on a Chrysler. It did have several codes, however, and a cylinder was filling with gasoline.

Heh, good old industry. What seemed like a good idea now looks like a dumb idea after a better idea came along.