Diagnosing High Idle Issue

[knedlin12]

Hello All,

I recently bought a 1998 Dodge Ram 1500 Laramie, 4x4, with the 5.9L gas engine. It only has 98,000 miles. I lived a bit aways from the seller so we agreed to meet half way. Long story short, met the seller, truck ran and drove great(truck was warm from him driving it to the meeting location). Bought it and came home. A few days later, went to go and start the truck and it has a really erratic idle when it first started. Sounds like it has a really loppy cam in it. Anyway, it will clear itself out after about 30 seconds and idles great afterward or once the truck is warm. It then idles fine the rest of the day too. It only does this on cold starts. Some times it bad enough it will kill itself out and you'll have to hold on the gas for a few seconds to keep it running.

I took the throttle body apart and gave it a really good cleaning. It was super dirty and gummed up. I believe the port to the IAC was almost clogged. I got the throttle body pretty much spotless, all ports cleaned and cleared, and cleaned the IAC. Start the truck up and immediately the trucks idles great on cold starts, no erratic idle, but I noticed its now idling at around 1,100 RPMs. I unhooked my battery and left the light switch on overnight to try and drain and reset the PCM. Put the neg cable back on the next morning and again, truck starts great and idles great but still idles at 1,100 RPM. Sounds like the PCM reset didn't effect anything.

Could I have damaged or changed the adjustment on the IAC when i was cleaning it? Also, the previous owner replaced the cat and both O2 sensors before I bought it. I also checked for vacuum leaks with starting fluid but did not find any.

Any ideas?

Thanks!

 

[Black-Wolf]

I believe it's normal to idle at 1100 RPM - esp. when cold - mine does. It should drop slightly when warm to about 1000 or a little below.

 

[knedlin12]

I though that too but it stays at 1,100 RPM even after the truck is completely warmed up. I though normal idle for these truck was around the 600/700 RPM range.

 

[Black-Wolf]

I though that too but it stays at 1,100 RPM even after the truck is completely warmed up. I though normal idle for these truck was around the 600/700 RPM range.

Well - I've never noticed it drop below 900-1000. I think that 600-700 would be a little low based on my experience. Only time I'd get worried is if it starts going above about 1500 or so at idle. I'll keep an eye on mine for a couple of days and let you know what mine looks like warmed up on Monday or Tuesday.

 

[Mister Luck]

Hello All,

I recently bought a 1998 Dodge Ram 1500 Laramie, 4x4, with the 5.9L gas engine. It only has 98,000 miles. I lived a bit aways from the seller so we agreed to meet half way. Long story short, met the seller, truck ran and drove great(truck was warm from him driving it to the meeting location). Bought it and came home. A few days later, went to go and start the truck and it has a really erratic idle when it first started. Sounds like it has a really loppy cam in it. Anyway, it will clear itself out after about 30 seconds and idles great afterward or once the truck is warm. It then idles fine the rest of the day too. It only does this on cold starts. Some times it bad enough it will kill itself out and you'll have to hold on the gas for a few seconds to keep it running.

I took the throttle body apart and gave it a really good cleaning. It was super dirty and gummed up. I believe the port to the IAC was almost clogged. I got the throttle body pretty much spotless, all ports cleaned and cleared, and cleaned the IAC. Start the truck up and immediately the trucks idles great on cold starts, no erratic idle, but I noticed its now idling at around 1,100 RPMs. I unhooked my battery and left the light switch on overnight to try and drain and reset the PCM. Put the neg cable back on the next morning and again, truck starts great and idles great but still idles at 1,100 RPM. Sounds like the PCM reset didn't effect anything.

Could I have damaged or changed the adjustment on the IAC when i was cleaning it? Also, the previous owner replaced the cat and both O2 sensors before I bought it. I also checked for vacuum leaks with starting fluid but did not find any.

Any ideas?

Thanks!

It is difficult to diagnose issues especially on a vehicle that has little service history 98,000 or do you have records of other services besides the cats and the O2 sensors ?
You mentioned the throttle body being clogged .


Do you think the intake manifold might need new gaskets and a little refresher? The timing chain might be a little stretched but it sound like it was just daily driver point A to point B
I would inspect wear parts like the serpentine belt the belt tensioners and any oil leaks and the condition of hoses
If you post some pictures of the engine bay that can help educated guesswork instead of just replacing parts all at once.

How to Replace Intake Gasket Set 1994-2002 Dodge Ram 2500

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Do you have any OBD2 codes you are chasing ?
You can have parts stores sometimes test the diagnostic connector for free along with the condition of your electrical system alternator and battery.

 

[knedlin12]

Well - I've never noticed it drop below 900-1000. I think that 600-700 would be a little low based on my experience. Only time I'd get worried is if it starts going above about 1500 or so at idle. I'll keep an eye on mine for a couple of days and let you know what mine looks like warmed up on Monday or Tuesday.

Thanks! I haven't be able to work on the truck lately. I'm planning on throwing a new IAC in it since they're only $40 and see what happens. If no change then I'll just roll with it.

 

[Mister Luck]

Thanks! I haven't be able to work on the truck lately. I'm planning on throwing a new IAC in it since they're only $40 and see what happens. If no change then I'll just roll with it.

I’d grease that Oring before putting a new one in.

Idle Air Control Valve (IAC) - 5.2 & 5.9 V8 ZJ Jeep / Dodge

Replace your engine’s IAC yourself and save some money!NOTE:Don't drop anything behind the engine. LINKS:IAC Valve: https://amzn.to/3shnLGUWrench Set: https...

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