Truck idles rough at start up only.

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Sabinoerc

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There are reasonably priced diag tools available - I don’t have experience with them but have seen recommendations on this forum. A search would find the discussions.

For any vehicle ~30 years or newer some level of diagnostic capability is going to be needed if you want to maintain your own vehicle. In some ways, the OBD1/ALDL on these trucks are easier to diagnose than newer OBDII stuff as they are not as complex and the mfg didn’t hide all the important data under proprietary interfaces as they do on new cars.

It can definitely be frustrating at times but it’s kind of the way of the world in autos today unless you want to only drive and maintain pre-computer cars/trucks. I have a 93 suburban I bought new and started out just trying to replace stuff when it didn’t run right. It’s definitely paid off investing in time/$ to get capability to look at diagnostic data if you want to keep one of these on the road.
Good luck!
 

Brandon_Lutz

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Runs great but now idles at around 1700 RPM

Sounds like a vacuum leak. My truck started to idle like this a couple of years back, super high idle until I ran it bit. Turned out the grommet around the PCV has stiffened and was letting more air into the TB until the truck warmed up a good bit and then the rubber would expand enough to seal it off. Get some throttle body cleaner and spray around your vac lines while it's idling, if the engine stumbles/chokes down when you spray around a particular line or connection, that is your culprit.
 

JACK34

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I took EGR valve off and capped the intake with a plate I made and sealed it up, did away with the EGR solenoid and capped that vacuum line. Still runs the same, rough idle at start up, high idle around 1500 rpm but drives fine. Did that paper clip trick to get codes and the only one that came up is 33 the MAP sensor.
 

Schurkey

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What does the scan tool show for MAP voltage? How do you know the MAP wire harness isn't damaged, or the hose split/blocked screwing up vacuum delivery to the sensor?

As long as you have the scan tool connected, verify ALL the sensors and computer outputs.
 

JACK34

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What does the scan tool show for MAP voltage? How do you know the MAP wire harness isn't damaged, or the hose split/blocked screwing up vacuum delivery to the sensor?

As long as you have the scan tool connected, verify ALL the sensors and computer outputs.
Yeah I don't have a scan tool I used a paper clip. Vacuum line is new. Bought yet another IAC today and that solved the idle problem. IAC must have been from China. Have yet to drive it but so far so good.
 
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JACK34

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So I started it up this morning and it's still rough idle at a cold start. Took it about 5 to 10 minutes to really come out it and be drivable. After that it ran great drove around for about a hour idle is a little low so I went out to set the base idle and there is a plug in front of the torx idle adjustment screw. Does anyone know how to remove this plug? Any help would be appreciated.
 

Erik the Awful

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The plug is there for a reason. If you start adjusting the idle you'll just cover up the problem. You might get it running better in the short run, but it'll make more problems for you in the long run.

Schurkey's going to tell you that you can't get a good diagnosis without a scan tool, and he's not wrong.
 

JACK34

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The plug is there for a reason. If you start adjusting the idle you'll just cover up the problem. You might get it running better in the short run, but it'll make more problems for you in the long run.

Schurkey's going to tell you that you can't get a good diagnosis without a scan tool, and he's not wrong.
Ok if you have scan tool you like could you please recommend one. Everything I have been able to find on setting base idle is pretty much the same. Get the truck warmed up. Jump the 2 terminals, turn the key on, wait 30 seconds then unplug IAC. Start the truck up with terminals still jumped and adjust the idle screw until you have around 700-750 rpm. Shut truck off pull jumpers, hook IAC back up and you're done. They also said after that your supposed to adjust you TPS voltage by loosening the mounting screws and turning it until you achieve .5 volts, mine is at .6 volts and it doesn't adjust. Lot of people have opinions on this I guess. I appreciate the help. Thank you.
 

tayto

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i grind a flat spot on the convex shaped tamper plug with a small dermal, then drill with a #7 drill bit, then tap it with a 1/4-20 tap. as you tap it the tap will bottom against the idle screw and push the tamper plug out
 
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