Running rough after highway trips.

[mike33]

Was wondering if anybody had ideas on what may be going on with my 2002 Deville. It's always run beautifully, until recently it's developed a rough idle in park after a highway trip. You can see the RPM's go up and down between 700 or so and 500 or so. Never goes lower than 500rpm. just goes back and forth between those two rpm's. Recent items done are plugs, air filter, fuel filter and it has run great with these items until recently. For the last year or so, I have had the misreading fuel guage..don't know if that's connected. Thanks in advance, Mike

[BodybyFisher]

On my 96, I used to have problems after highway trips with high idle, the gunk from the rear of the throttle plate was sucked to the edges of the plate and the plate was held open

I would start by cleaning the throttle body good and focus on your IAC motor, you could be getting build up on the pindle on highway driving

[Cadillac Jim]

You might want to read out the OBD codes (see the link in my signature block for instructions) and post them here.

[BodybyFisher]

Good point Jim, see if you are getting a P0300, maybe this is a misfire? I was taken in by the 500 to 700 ranging idle

[Cadillac Jim]

The OBD codes would certainly help in narrowing down the cause, by eliminating - or focusing on - things like engine miss or idle air control problems. If there are no codes, we can focus on things like vacuum leaks and throttle body debris.

[mike33]

ok here are the codes history: U1713,B0159,B1004,U1016,P1567,C1287

this is the only current code: B0429

The car has 170k on it. The throttle body was cleaned at 150k, about a year ago. Also, ICA motor was replaced at that time. All has been well until last couple weeks. It does not like driving 45 miles at 70mph, then arriving at house and placing in park!

I let it sit for ten minutes, then restart, it does seem to smooth out a little. Time for another throttle body cleaning? Maybe the dealer lied to me and didn't clean it, ugh.

[Cadillac Jim]

B0249 Heater/Defrost/AC Door Range Error

U1713 Loss of Front Door Module to Window Motor High Speed Comm.

B0159 Outside Air Temperature Sensor Circuit Range/Performance

B1004 Keep Alive Memory (KAM)

U1016 Loss of Communications with PCM

P1567 Active Banking Control Active - Cruise Control Disabled

C1287 Steering Sensor Rate Malfunction

I don't see anything here that would cause poor idle speed control. So, it looks like a vacuum leak, a dirty throttle body, or a problem with the IAC stepping motor.

Vacuum leaks can be sneaky. Look at the PCV valve and its hoses, make sure that the dipstick is seated firmly, and that the oil filler cap is tight. Then check out the edges of the intake manifold for leaks and the gaskets for the throttle body. Check out the hoses that go to the throttle body and the intake manifold, all the way to the other end. If in doubt, close off a hose to see if that stops the problem.

But it is beginning to look like a dirty throttle valve or throttle body.

[joeb]

I saw a newer car at junkyard with intake gone. The plastic plenum and TB were still in place. The plenum had a huge tear in bottom. I could see light thru it. How this car even idled was a mystery. Maybe that's why it was in junkyard.

[BodybyFisher]

I saw a newer car at junkyard with intake gone. The plastic plenum and TB were still in place. The plenum had a huge tear in bottom. I could see light thru it. How this car even idled was a mystery. Maybe that's why it was in junkyard.

The only problem with that thought is that he is not getting the lean codes on both banks what are they P0171 and P0174?

I would check for a bad head gasket, a bad coil (test the primary and secondary coil circuits for anomalies) maybe a coil overheats (however this is not so easy with coil cassettes), I would check the spark plugs for cracks, carbon tracking and platinum tips as a start.

[Logan]

I'm thinking the rubber intake plenum is leaking. I think they will all fail sooner or later...maybe not bad enough yet to set the 171 and 174.

[BodybyFisher]

Thanks Logan, problems that don't reach the DTC threshold are hard to pin down

[mike33]

Well, I would like to close out this topic with my results. Thank you all for your help.

I cleaned the throttle body and installed a new IAC Valve. It did not work. At that point I just took it to the dealer, I figured it was beyond my capability. Turns out it was the rubber intake plenum that was leaking. So, good call Logan.

Runs smooth as silk now. So, if you have a older high mileage Northstar not a bad idea to take a look at the boot on plenum.

So, I have a nearly new IAC Valve if anybody needs one! I'll probably just hold on to it and install it on my 2008 DTS when it reaches 150k LOL.

Best Wishes, Mike

[BodybyFisher]

Good call Logan!

Mike, cleaning the IAC would have been sufficient, especially given there was no code. Glad you got it fixed, this is another instance where a DTC was NOT set as the problem was enough to create a problem but not set a code....

[Logan]

Mike33...I am curious....what did your dealer charge to diagnose and replace the rubber intake coupler?

[mike33]

The charge with labor, part, tax, and good ole recycling fee was about $380.00. That's all I have paid a mechanic for repairs in the last 14 months. So, I'm doing ok on this '02 with 170k miles. I try to do most things myself if I can handle the task.

Just curious.. do we have a lot of over 200k mileage NS's out there?

[Texas Jim]

My 2006 only has 122,000 on it.

I hope to get 200,000 out of it, at least...