Hi, I have a 1985 IROC with the 5.0 TPI motor with automatic trans-the car only has 11,000 miles on it.

It idles smoothly when cold, but I notice stumbling when the engine reaches operating temperature at idle.

Any ideas on what the problem areas are, and what I can do to check the issue out? Thanks in advance, Tom.

 

'85s are a bit unique because of the control scheme and system. It was the ONLY year that system was used. The ECM tends to be a bit slower and less capable, but it is still adequate.

One thing to consider is that when the engine coolant reaches temperature and the oxygen sensor becomes active, the system enters closed loop mode and relies upon inputs from sensors instead of lookup table for fuel and spark parameters. The critical sensors are oxygen and CTS. The MAF and TPS are active even on cold startup, so they would likely have less effect on the transistion to closed loop.

Even at only 11K miles, the oxygen sensor may be almost 25 years old and not producing adequate signal. Fortunately, replacements for a 1985 are a lot less costly than those for later vehicles. You can also test the sensor's output to see if it is still viable.

Another factor is the A.I.R. system. If the A.I.R. port/divert valve is not shifting when the engine warms up, the oxygen sensor will get an artificially lean signal causing the ECM to heavily enrich the mixture. Make sure the system is working.

A vehicle that old with that little use could have many issues like this. My '86 has a whopping 48K on it, and has suffered similar problems from the longer periods of sitting unused, even though I try to start it at least once a month. I've cleaned the injectors twice, replaced a heater core, and replaced tires twice just from sitting most of the time.

 

That is very helpful. Wondering what the CTS is, and also how can I check the A.I.R port/diverter valve? Is there a way to test the oxygen sensor? Much appreciated!

Also wanted to mention that the car runs well off idle, although it does seem to have less power than I would expect compared to the one I used to have (same 5.0 TPI) although we are in Reno at 5,000 feet vs. Ohio at 500 feet

 

Coolant Temperature Sensor

The O² can be tested in an open propane flame. Expect to read at least 0.90VDC from the sensor after it has been heated and is completely in the propane flame, then a rapid jump to less than 0.35VDC once the flame is removed. obviously, the sensor has to be removed for this test.



A replacement is about $25.

 

The Air Injection diverter valve can be checked with the air lines removed. As a temporary test, you can simply remove the output hose and cap the line so that no air can reach the exhaust manifolds to dilute the O² sensor gasses.

 

Hmm, never thought of the O2 sensor...Having issues now, no power, won't idle, no power under throttle. Just started today. Running good, came to a stop light, took off, bam, almost no power...completely sucks. Not sure where to even start.