Irregular idle on startup

[intheSFValley]

Hey everybody,

I've got a minor issue I'm trying to diagnose:

When I start cold, the engine idles fine for 30-45 seconds, then the RPM dip and fluctuate (like its about to stall) for 10-15 sec, then it goes back to a good smooth idle. It does the exact same thing if I drive around, stop for 15-20 minutes and restart. If I restart within a few minutes, no problems.

Its a 1997 w/2.5L and 122k , no CEL, runs well and just passed smog w/great numbers. I've changed the plugs, wires and cap.

My first idea was that it was a bad fuel pressure check valve/regulator, but it seems like the exact same amount of time cold or warm, so probably not a fueling/vapor lock issue (?)

I'm thinking either an O2 sensor getting warmed up, or maybe an IAC/throttle body needing cleaning?

any ideas on a fix before I throw parts at it?

thanks!

robert

 

[Jerry Bransford]

Could be a bad upstream O2 sensor but that can also be caused by a dirty IAC (idle air control) which is what meters/provides all of the engine's air at idle rpms.

How to clean the IAC: Buy a can of throttle body cleaner then remove the air intake tube from the throttle body. Start the engine and with the engine at idle rpms, spray the cleaner into the throttle body until the engine stalls. Don't raise the engine rpms to try to prevent the engine from stalling, that would prevent the cleaner from reaching the IAC which only draws air (and thus the tb cleaner) through itself at idle rpms. Keep spraying the cleaner into the throttle body at idle rpms until 1/4 to 1/3 of the can has been used. The engine will repeatedly stall, that's ok just restart the engine. The engine is stalling since the cleaner is flooding the interior of the IAC and blocking the air flow which is normal and actually good. The more time the cleaner sits inside the IAC the more time it has to clean the black gunk out that forms inside from normal engine operation. Once 1/4 to 1/3 of the can has passed through the IAC at idle rpms, you can raise the engine rpms and spray the rest of the can of cleaner down inside the throttle body to finish cleaning it.

The below illustration shows the IAC's air control passage inlet the IAC draws all its air through, you can aim the cleaner toward that slot. The bottom photo shows the IAC itself, it has a plunger that moves in/out of an orifice in the throttle body to meter the amount of air the engine gets at idle rpms. The IAC does nothing once the rpms are raised up off idle which opens the throttle body's throttle plate to let air into the engine. Removing the IAC to clean it is sometimes done but that doesn't also get the orifice it fits into clean. Cleaning the IAC while it's installed as described above is usually entirely effective at restoring the IAC back to full-normal operation.

If that doesn't fix the idle issue, the next thing I'd suspect would be the upstream O2 sensor. If you end up replacing it, make sure to ONLY install an NTK sensor. Avoid Bosch O2 sensors which are well known to not work well in our Jeeps.

 

[intheSFValley]

thanks, good to have confirmation of what I was going to do next. I'll try these and post the results!

 

[jwrangler187]

For some reason they get really smogged up. I had an issue with my 2.5. Cleaned the intake and seemed to work better.

 

[intheSFValley]

I cleaned up the IAC , and it does run smoother; however, the idle speed moved from 7-800 to 1200 now. Might end up replacing the (original factory) unit and seeing if that does it.

While I was there, I found my vac leak! The hose might have been cut at some point. Here's my issue now, where does this hose (which runs along the firewall) attach to? I can't find any ports on the intake manifold that are open.

 

[Jerry Bransford]

I cleaned up the IAC , and it does run smoother; however, the idle speed moved from 7-800 to 1200 now. Might end up replacing the (original factory) unit and seeing if that does it.

While I was there, I found my vac leak! The hose might have been cut at some point. Here's my issue now, where does this hose (which runs along the firewall) attach to? I can't find any ports on the intake manifold that are open.

View attachment 70746

A high idle rpm is usually caused by a vacuum leak.

 

[intheSFValley]

I've found the port for the brake booster, and one that returns to the crankcase cover, and one for the fuel vapor system, and one that goes to the fuel pressure rail.

Am I missing something?

I keep looking for the other end of that hose and cursing the mouse that chewed it.

 

[intheSFValley]

and found it. There's a fifth port on the intake manifold which points upward. Almost by the firewall.

Which also explains why the vent doors don't move.

 

[intheSFValley]

Wrap up: Here's a shot of where the heater line goes into the intake manifold. A bit hard to see if you don't look behind the plastic body above the throttle body.

repaired the broken line with hoses and shrinktube, replaced the throttle body gasket and cleaned the throttle body and IAC. idle back to normal, all vent doors working.

 

[RicanCrazy]

the cut line is supposed to go to the intake manifold. should be a rubber L attached to it and that line going to the cut. It runs from there to the fire wall and T's. The T goes into the firewall and the other side goes below the battery box to a vacuum container. Only reason I found all this out recently is because I've been tracing a goblin myself that may or may not be vacuum related.... Good luck!