1990 Oldsmobile 98 Runs rich

Tiny
CHARLIE1965
  • MEMBER
  • 1990 OLDSMOBILE 98
Engine Performance problem
1990 Oldsmobile 98 6 cyl Front Wheel Drive Automatic

Thank you for your time. I have a 90 Olds 98, when it's cold it runs rich at start up. The service engine, and temp light come on during the rough idle. But it has NO codes, current or history, not even while the light is on. Fuel pressure within specs (40-48), regulator checks out good. Replaced module, crank sensor (was dieing), and computor.
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have the same problem?
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No
Wednesday, December 10th, 2008 AT 9:34 AM

20 Replies

Tiny
JAMES W.
  • MEMBER
A bad MAP sensor will make it run rich on cold startup. But, I would be more concerned with the ECM first. In your posting you "replaced module", which module? Please advise.
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Monday, December 15th, 2008 AT 2:05 PM
Tiny
CHARLIE1965
  • MEMBER
The module I replaced was the one under the coil pack. I have also replaced the temp. Sending unit. The ECM has been replaced. Running out of options. Thank You for your responce.
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Monday, December 15th, 2008 AT 3:15 PM
Tiny
JAMES W.
  • MEMBER
Well, without engine codes to go by, I would have followed the same routine you have. I'm going to give you a few assumptions.
Then you start the car cold, it "chugs" until it warms up, then it runs fine. Yes?
At a cold start, if you rev it up a little, it kinda' clears up a little, but back at an idle, it chugs again. How am I diong so far?
A theory, engine temp sensor is telling the ECM the engine is cold and richens up the fuel pulse to compensate. It should also open up the idle air control valve to compensate for the richer mixture, but it's not. Have you cleaned the IAC control or checked it for operation? Please advise.
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Monday, December 15th, 2008 AT 4:49 PM
Tiny
CHARLIE1965
  • MEMBER
Thank You,
You are dead on so far. I have not checked IAC. Car does not have MAP sensor, so I guess we can rule that out. I will chech IAC. Any idea why Service engine, and temp light are on while it chug's?
Thanks again,
Charlie
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Monday, December 15th, 2008 AT 8:14 PM
Tiny
JAMES W.
  • MEMBER
Charlie, if you raise the rpms or after the engine warms up, do the lights go off? They both get their signal from the ECM. If they are on all the time, I would suspect a problem in the wiring between the dash panel and the ECM. Let me know.
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Monday, December 15th, 2008 AT 8:36 PM
Tiny
CHARLIE1965
  • MEMBER
Lights go out when engine runs better, like you said, when engine is cold it chug's (runs rich), and engine / temp lights are on.
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Tuesday, December 16th, 2008 AT 8:21 AM
Tiny
JAMES W.
  • MEMBER
Charlie, if the IAC valve is just "grunged up", you can clean it with a can of spray carb cleaner. This seems to work the best.
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Tuesday, December 16th, 2008 AT 1:17 PM
Tiny
CHARLIE1965
  • MEMBER
I took throttle body off cleaned entire throttle body and IAC. Still have same problem on cold start up (chug's). Throttle body was very grunged. Sounds like it has a miss. It sounds like it's missing out the tail pipe.
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Friday, December 19th, 2008 AT 11:35 AM
Tiny
JAMES W.
  • MEMBER
This is a "toughy". With no engine codes, let's try to create one. Try unplugging the O2 sensor and see if the computer set a TC for an O2 problem. I think if we find out why the ECM isn't registering codes, we'll find the problem. Let me know. Also, if the motor is running as rich on startup as you say, you may have fouled a plug or 2.
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Saturday, December 20th, 2008 AT 3:27 PM
Tiny
CHARLIE1965
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Since we talked, I unplugged TPS, and got a code. I have also replaced plugs & wires. Still have same problem. Chug's on cold start, with Service Engine and Temp. Lights on. Still no codes, other than TPS (that I unplugged). Still sounds like a miss, when chugging. Runnng out of idea's. Please tell me of anything we've missed.
Thank's
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Monday, December 22nd, 2008 AT 10:23 AM
Tiny
JAMES W.
  • MEMBER
Everything still points to the IAC. With engine cold, pull the IAC out. When you turn the key to "on", the IAC motor should pull the valve back, or to open. If not, do you have voltage to the connector and continuity through the motor?
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Sunday, December 28th, 2008 AT 1:57 PM
Tiny
CHARLIE1965
  • MEMBER
I tried turning the key on with the IAC out of the throttle body. It kinda wiggled, (didn't really move). But when I turned the key off, it came all the way out. The plunger fell out into my hand. I tried a used one I had (to a different year motor), and it did the same thing. Where do we go from here?
Thanks
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Monday, December 29th, 2008 AT 8:55 PM
Tiny
JAMES W.
  • MEMBER
OK, when you start the cold, and while it's doing it's chugging thing, pull off a vacuum line somewhere on the intake manifold so you create a gross vacuum leak. Your checking to see if the rpms rise and the "chugging" improves. In doing this we are doing the same thing as the IAC does.
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Tuesday, December 30th, 2008 AT 7:21 PM
Tiny
CHARLIE1965
  • MEMBER
Yes, RPM's increase with vacume hose disconected.
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Tuesday, December 30th, 2008 AT 9:29 PM
Tiny
JAMES W.
  • MEMBER
Does the "chugging" seem to be less with the vacuum hose disconnected?
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Tuesday, December 30th, 2008 AT 9:45 PM
Tiny
CHARLIE1965
  • MEMBER
Yes, RPM's come up, and it runs smoother. Is the IAC working correctly? Or do we need to replace it? As I stated, it wiggles when I turn the key on, and push's plunger all the way out when I turn the key off.
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Wednesday, December 31st, 2008 AT 9:58 AM
Tiny
JAMES W.
  • MEMBER
There has to be a problem with the wiring between the ECM and the IAC unit. Do you have, or have access to a fax # and I can ship you a copy of the diagram. I can attach it to this post, but it won't come through very clear. There are 4 wires connecting to the IAC. One open circuit and one to close. Apparently yours is closing alright, but it won't open. Please advise. I wish I had a diagram for the intermnals of the ECM, but I don't.
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Wednesday, December 31st, 2008 AT 1:08 PM
Tiny
CHARLIE1965
  • MEMBER
Thanks,
I have access to wiring diagrams. I will check this out, and let you know what I find.
Thanks
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Wednesday, December 31st, 2008 AT 5:34 PM
Tiny
CHARLIE1965
  • MEMBER
Thank You for all your help. FINALLY got it fixed. Replaced crank sensor AGAIN, must have gotten a bad one. Cranked right up, and idled smooth as can be.
Thanks Again.
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Wednesday, January 14th, 2009 AT 2:47 PM
Tiny
JAMES W.
  • MEMBER
Charlie, the only thing I can think of is the CPS must have been sending a late signal to the ECM therefore, retarding the ignition timing and injector pulse. Something in my guts tells me this is going to come back to haunt us, I hope not. Glad you found it. I'll keep this one for future reference.
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Wednesday, January 14th, 2009 AT 8:23 PM

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