• NAXJA is having its 18th annual March Membership Drive!!!
    Everyone who joins or renews during March will be entered into a drawing!
    More Information - Join/Renew
  • Welcome to the new NAXJA Forum! If your password does not work, please use "Forgot your password?" link on the log-in page. Please feel free to reach out to [email protected] if we can provide any assistance.

Erratic idling

bigalpha

Moderator
Location
Tucson, AZ
I know, millions of threads about this. I searched and read but still unsure of my next move.

Specs:
- 88 MJ
- AW4
- I6
- 160,xxx miles

Symptoms:
- Truck idles high in 'D', but the idle is steady.
- I don't have tach, so unsure of RPMs.
- When shift from 'D' into 'P' or 'N', the idle will shoot up.
- Sometimes, the idle will surge. Sometimes it is not so bad.
- I have a weird blinker issue, but not sure if related.

Troubleshooting:
- No discernible vacuum leaks.
- Replaced the IACV with new but it made the problem worse.
- Original TPS tested out good but I replaced with new and adjusted.
- Fixed the serious blow by issue.
- Grounds test out .. tested via hot on batt to ground.
- Oil Pressure steady.
- When shift from 'P' to 'D', the truck engages gear and leaps forward. Same when shifting to 'R'.
- Short lag when trans downshifts. Engine will very briefly rev up before downshifting.
- Truck will accelerate on it's own and upshift through gears, maintaining about 45 mph.

Questions:
1. I think the PO adjusted the idle screw on the TB. If so, this can account for the high idle in 'D'. How do you get that back to proper setting?

2. If I unplug the IACV, should the idle speed change? Same with TPS?

3. If IACV and TPS test out properly, but the TB is really dirty from blow by, can that affect idle?
 
Clean the TB thoroughly, and check the plate mounting screw are not loose.

Disconnect the throttle linkage.

Back out the base idle screw so that it no longer engages and the throttle plate is fully closed, flip the throttle a couple times to make sure it is fully closed, then adjust the base idle screw in until it just barely engages and you can observe a slight movement of the throttle plate.

Reattach the throttle linkage making sure it doesn't move the throttle.

You should have an idle speed of 700-750 RPM in Drive. Don't trust the Parking brake or wheel chocks to hold the vehicle or you will run yourself over or cause property damage, you need someone in the driver's seat with their foot on the brake, and the Parking brake set.

Beg, borrow, buy, or steal a tach.
 
Clean the TB thoroughly, and check the plate mounting screw are not loose.

Disconnect the throttle linkage.

Back out the base idle screw so that it no longer engages and the throttle plate is fully closed, flip the throttle a couple times to make sure it is fully closed, then adjust the base idle screw in until it just barely engages and you can observe a slight movement of the throttle plate.

Reattach the throttle linkage making sure it doesn't move the throttle.

You should have an idle speed of 700-750 RPM in Drive. Don't trust the Parking brake or wheel chocks to hold the vehicle or you will run yourself over or cause property damage, you need someone in the driver's seat with their foot on the brake, and the Parking brake set.

Beg, borrow, buy, or steal a tach.

Cool, thanks. So I need to perform this procedure with the truck running?
 
Only the part where you are using a tach to check the idle speed in drive, all else is engine off.
 
Alright, thanks.

Can a dirty TB cause erratic idling even if the IACV and TPS are functioning properly?
 
Yes. If the throttle plate is sticking in an open position then the IAC and TPS are going to be "out of range" and trying to compensate.
 
Here's another piece of the puzzle. I pulled off the weird tailpipe endpiece that the PO put on .. I guess it makes it sound better? :dunno:

Anyways, it's all black. Really black. Is that a sign of running rich?
 
Here's another piece of the puzzle. I pulled off the weird tailpipe endpiece that the PO put on .. I guess it makes it sound better? :dunno:

Anyways, it's all black. Really black. Is that a sign of running rich?

Yes. Is your thermostat 195 degree? If it isn't you will run rich. Is your O2 sensor functioning? Is your coolant temp sensor (not for the gauge/idiot light) functioning?
 
Honestly, I"m not even sure that there is a t-stat installed. It's either missing or stuck open. I am pricing heater coolant hoses so I can replace it all at once. The hoses are in pretty poor shape. I don't want to do this twice.

The dash gauge shows much less than 210 (I know this can be inaccurate and not the 'official' gauge). However, the dash gauge is consistent. I also get hot heat when it 'warms up'.

Not sure about the coolant temp sensor values. I have not crawled up underneath to test it. The MAT shows proper values, however. Not sure if they'll be similar; I know they use the same values.

I am not sure about the 02 sensor. I've read some stuff that says it's pretty hard to accurately check it. With the amount of blow-by this thing had, it's possible that the 02 sensor is fouled.

Could these things cause the weird idling?

Recently, the idling has become more erratic. It'll idle high, then drop down to what I assume is normal. Then, slowly build up real high.
 
New symptom: I got in the truck teh other day in the morning, started it in 'P'. When I shifted into 'R', it died. Just bogged out.

Today, same thing happened but instead of dying, it bogged down for 2-3 seconds then started running alright.

What's the relationship between shifting gears and the engine bogging down?
 
I chased erratic idling on my 88XJ awhile back. After tightening the manifold bolts, cleaning throttle body, etc. I still had the problem after warm-up. On close inspection, I found a cracked intake manifold that the previous owner had very cleverly sealed with aluminum epoxy. This bandaid fix was tough to spot. When the engine heated up, the crack opened up and more air entered the engine to increase the idle speed. Replaced the intake manifold and all was well. You might want to spray WD40 around the intake manifold to see if any is drawn into the engine.

Hope this helps.

Best regards,

CJR
 
I think the easiest is to use the unlit propane torch method. Open the valve and point the torch around where you think you may have leaks. Listen for a change in idle speed. This was demonstrated on youtube if you want to see it.
 
Well, looks like something to put on the shopping list. Will the idle speed change even if it's idling really high to begin with?
 
Read my old thread, search for "RenX Files" here for all the causes and solutions, other than loose intake manifold bolts, and cracked intakes.
 
I have an intermittant High Idle also, when I disconnect my TPS it goes away.

That is typical. With no TPS it reverts to just the CPS data for engine speed, but you lose throttle response with out the TPS.
 
Back
Top