Tech Stuff

Tracing poor engine idle

A faulty IAC (idle air control) will definitely cause idle issues. A Ford Ranger example is seen here. If a timing belt jumps, this can quickly take out the IAC.
<p>A faulty IAC (idle air control) will definitely cause idle issues. A Ford Ranger example is seen here. If a timing belt jumps, this can quickly take out the IAC.</p>

Engine idle issues can obviously involve a wide range of potential faults, involving ignition, fuel, vacuum and mechanical problems. In this article, we’ll discuss a variety of potential sources that result in poor engine idle characteristics. (See the list at right for potential causes.).

We’ve included both general tips as well as a few case-study examples of how “mystery” idle problems were addressed in specific cases. Our thanks to both Craig Truglia and Greg Montero for their invaluable insights.

A case study

2001 Mitsubishi Montero 3.0l rough idle

By Craig Truglia, Owner & Service Manager, Car Clinic Inc., Mahopac, N.Y., and Associate at Technician Service Training

This was a tough one. A 2001 Mitsubishi Montero equipped with the 3.0L engine came into the shop with a mysterious idle problem. The engine ran really rough at idle when it was cold. It had a non-specific P0300 random misfire code. It had a DIYer as its owner, so we also had to consider all of the potential areas that he had tampered with. We knew we were in for a fun one, but had no idea how many things we would have to chase down in order to get it running normally again.

In the beginning, nothing appeared that suspicious. A DIYer, who otherwise only brought us inspections, had a problem that he could not fix himself. He told us that it ran really rough when it was cold. The only code it has is a P0300 random misfire and it never had new spark plugs. Because he had to remove the intake manifold to do the tune-up, he gave the car to us.

When we pulled out the first plug we saw that it was fouled with gasoline. The customer demanded the car back ASAP, so we were not given the time to ponder why the spark plug was wet. We returned him his car and hoped for the best.

A few days later the customer complained that the car was doing “the same thing.” We expected to get the car back, but nothing came of it.

Then, a couple weeks later, unannounced a green Montero was parked behind our shop that morning. We started it up and this time it was running horrifically. Only scatological terminology would really explain how the quality of its idle at this point.

In diagnosis, generally we are always looking to trace down the root cause of everything, what the Eagles’ Don Henley may call “the heart of the matter.” We presumed the vehicle either had an ignition, fuel or mechanical problem. What are the chances it’s more than one, right?

First, we wanted to see if the engine was not getting proper fuel. We checked fuel pressure (41 psi) and volume (0.3, a little low). Then, we wanted to make sure that the plugs were getting proper kV and all its ignition coils had good waveforms on our labscope. To be honest, I don’t know what a good Mitsubishi coil should look like. But I do know that most coils have similar waveforms, so I played the “comparison game.” I simply checked what all six coils looked like on my eight-channel labscope and they all looked the same. After not finding any obvious fuel or spark-related problems, we did a relative compression test to rule out an engine mechanical issue.

Personally, we find that on some weird cars (Mitsubishis, Subarus, and even Hondas) relative compression with a labscope is not an effective test. This vehicle tested really well, so we were sort of dumfounded. Obviously, something was wrong, this car was idling really bad.

Sometimes I feel like auto repair is like an arms race. It was time to pull out the big guns. So, we used the ACE Misfire Detective and it reported misfires and a relative compression issue (this particular tool makes measurements at the tailpipe). Now, we had good reason to suspect the engine had a mechanical issue. We put a pressure transducer into number 1 cylinder to check compression and timing.

We hit the nail right on the head! The pressure transducer revealed that the timing was really bad. The Automotive Test Solutions EScope makes it really easy to interpret.

Read a pressure transducer waveform just like the following: At 0 degrees top dead center the BANG or Power Stroke is occurring. The line is up high because the cylinder is at its most compressed when the spark event occurs. At 180 degrees bottom dead center the exhaust valve opens as long as engine timing is correct. When at 360 degrees TDC the piston begins going down and the cylinder decompresses (the SUCK) begins. At 540 degrees BDC the SUCK completes and the SQUEEZE begins. At 720 degrees TDC the process repeats.

If the waveform shifts in either direction left or right, timing may be advanced (left) or retarded (right). Other irregularities reflect that the car is not sucking, squeezing, banging or blowing correctly.

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