Code 551 IAC circuit failure

[ahelm]

I have a 93 mtx sho that has code 551. The car runs but sometimes becomes sluggish and the engine light will go on, then it will run normal and the engine light will go off. the problem is intermittent, but common and I did replace the IAC valve, but that didn't help. Any suggestions?

 

[SHOdded]

No other codes? Is the sluggishness at idle, while on the road, etc? Any vacuum leaks? Any other work done on the car recently (thinking loose/pinched wiring/hoses or improperly torqued nuts & bolts etc.)?

 

[ahelm]

The sluggishness is when I'm driving and sometimes at idle. When I do the diagnostic test with the car running I get codes 538, 536, & 521. No work has been done to the car recently.

 

[SHOdded]

Codes can be checked here: http://www.shophoenixproject.com/eec/code3.htm

Seems like the 536 code for the BOO switch is the only useful code of the 3. But I don't see why that would cause sluggishness. I did have it fail on me once and I could not start the car until it was replaced. If this code is repeatable, then replacing it is worth a try.

 

[rubydist]

iirc, the tps, mafs, and act sensors are on that same circuit as the iac. so if the pcm is getting bad information because the iac circuit is bad, it is also likely getting bad information from the tps, mafs, and act sensors. The motor will not run right if the pcm is getting bad information from them.

The solution is to get rid of the 551 code - it may be as simple as cleaning connectors, or it may be a wiring harness issue. As these cars age, I have seen more of the problems where a wire is bad somewhere in the harness - voltage at one end does not appear at the other end. That sort of thing will make the pcm crazy and cause the engine to run poorly.

 

[ahelm]

Thanks. I'll let you know what I find out....

 

[ahelm]

So I checked all the connectors, but I don't know what the iirc is. I was able to check the ACT and IAC sensors with my voltmeter, but I'm wondering if there is another way to check the voltage for the MAF and TPS and the other harness wires without having to buy the PCM breakout box. Any help would be appreciated..

 

[SHOdded]

iirc= shorthand for "if i remember correctly"

 

[ahelm]

So is there a way to check to voltage of the wiring harness without buying the breakout box?

 

[rubydist]

probe the various connectors with your voltmeter.

 

[ahelm]

So I solved my problem. It ended up being the DIS module. Once I put a new one in the car has been running great!