just a couple of questions when scanning codes

[88mercgrandmarq]

I hooked up my code reader to my 88 gm today and theres a couple of codes that came up that I have questions on. One was code 12 that said cannot control rpm during self-test high rpm check. Does anyone know exactly what this means? And the other was code 33 which was EGR valve opening not detected. Would these be serious issues and what would all be involved in getting these fixed?
Any help would be greatly appreciated!

[Lincolnmania]

12 is usually a bad idle air control valve

33 is usually a broken vacuum line somewhere in the egr plumbing

[88mercgrandmarq]

Would the bad IAC cause it to idle weird? Lately when I start it in the mornings it'll idle high and will also surge for a few seconds and then will go down to normal. Also, it will surge at idle after the engine being warm and ran for a while.

[88grandmarq]

Code list is here; http://www.grandmarq.net/vb/showthre...-trouble-codes

Scott is correct, code 12 is usually a faulty IAC or ISC motor... replace it.

code 33 could be a broken vacuum line or a faulty EVR solenoid.

[88grandmarq]

Would the bad IAC cause it to idle weird? Lately when I start it in the mornings it'll idle high and will also surge for a few seconds and then will go down to normal. Also, it will surge at idle after the engine being warm and ran for a while.

Yep, it will cause that... Replace it.

[88mercgrandmarq]

I went out over my lunch hour and drove the car around some and got it up to operating temp, I unplugged the IAC connector and the car stalled instantly. I figured since it stalled like that it should still be working or could it still be bad? And could you tell me what an ISC motor is? And I checked all the vacuum lines under the hood and they looked fine and everything was plugged in right.
This car has been nothing but a headache this past few days haha

[1990LTD]

ISC = idle speed control - just another of the many names for IAC. Idle air bypass valve is another one. Throttle bypass valve etc etc


It sounds like the IAC problem might be intermittent. I'd replace it with a Motorcraft (junkyard ones have proven to be better than new cheapie ones)

It would be a good idea to clean it (and the throttle body) before you install the replacement.



Other things to look at regarding the cold start idle - coolant temp sensor, air temp sensor.

[88mercgrandmarq]

thanks I'll see if I can get one tomorrow and see what happens!!

[88mercgrandmarq]

i was thinking back to this problem i had earlier this month and when i was digging through my tool box in school today i found another egr valve off a different 5.0. I thought what the heck and put this one of my car to see what would happen. I disconnected the battery while changing the valve to clear the codes, and when I started her back up I connected my code reader and got the following codes- code 12 went away completely, code 33 went away also but code 34 came up about the evp sensor, I had the other evp sensor from the old valve yet so i put that on, then code 34 went away but 33 came back on!!! also my code 91 for running lean came back and also code number 41 came back for running lean. This is driving me up a wall!!!! Does anyone have any ideas??

[86VickyLX]

Pull the IAC, clean it. It's probably full of oil. 33 is broken vacuum line most likely.

[88mercgrandmarq]

earlier this year I had the car hooked up to a smoke machine to find vacuum leaks and I fixed the ones that were there and all the lines on the look to be in good shape yet. Would a dirty IAC really cause this many problems?

[86VickyLX]

earlier this year I had the car hooked up to a smoke machine to find vacuum leaks and I fixed the ones that were there and all the lines on the look to be in good shape yet. Would a dirty IAC really cause this many problems?

Yes. And you can't smoke test things that are closed when the car isn't running. Green line.

[Phoenix]

earlier this year I had the car hooked up to a smoke machine to find vacuum leaks and I fixed the ones that were there and all the lines on the look to be in good shape yet. Would a dirty IAC really cause this many problems?

For the IAC, don't buy Duralast from AutoZone. I would frankly clean yours first before buying anythiing. Look up discussions on how to do it at any of the 5.0 Mustang forums, they're all talking about this all the time. YES, all of these 302 cars will idle HORRIBLY with a bad IAC, take it from a Mk7 guy, I went through a bunch of um until I found one that worked. One stupid little bolt-on part, but what a difference. Remember - aftermarket IACs these days mostly suck, so my solution actually proved to be a used IAC that just turned out to be good.

I'd hit the yards if cleaning yours doesn't solve it and grab like a dozen of um, then try them all cleaning each one then disconnecting the battery each time for about 30 mins to reset the PCM, until you find one that doesn't idle like crap. Mostly likely it won't code 12 either.

For code 33, in my case it was the EGR solenoid. If you have no vacuum leaks, try replacing that. I got an AutoZone Duralast solenoid on my car and it works fine.

Good luck!

[88grandmarq]

you can't smoke test things that are closed when the car isn't running. Green line.

Pure Genius !