Erratic high idle help, please!

[Maui-sprint]

Im currently having a similar issue with my rig as well, however, doesn't appear as bad as yours. My truck will idle high when I come to a stop light or have it in neutral when warm. Idles at about 2000rpm. But not always! Sometimes (rarely) it will idle perfect around 800 rpm. So my issue comes and goes as well. Motor runs smooth as silk other than the high idle.

I performed all the same checks and replaced most of the sensor: IAC, MAF, TPS, PVC, & O2 (only one on my truck). All to no avail. Have not changed out the throttle body yet but its next! No vacuum leaks to speak of. Im not too certain that the new IAC is doing its job since I get no change in rpm at idle when I disconnect the pigtail. Is it supposed to shut CLOSED when disconnected? My truck was originally an AUTO but I converted to a MT but did not swap out the PCM. I'm told that this is only good for a few hundred RPM so didn't want to spend the $$$ just yet.

1991 XLT 4x4
4.0
Auto to manual conversion

 

[ActuallyaMazda]

These idle gremlins are hard to chase! It acts like a vacuum leak, but if it were a vacuum leak it’d be doing it any time it is drawing air through and not intermittent! Super frustrating.
I installed 2 new O2 sensors and the backfiring and rough start have all but gone away, but the idle still climbs high after a PCM reset. I might order a refurbished one at this point and see if that works. I am also going to try changing the MAF out because if I unplug it, it idles normal, but the moment I give any throttle, it tries to stall.

 

[Maui-sprint]

How did you do the PCM reset? I haven't inspected mine yet.
Im down to three things: PCM, IAC, and replacing throttle body with a rebuilt unit. Im living with this now cuz I choose to spend my $ on higher priority items haha

 

[franklin2]

Do your engines have EGR valves? What if they are opening and then not shutting correctly? That would cause a vacuum leak.

 

[ActuallyaMazda]

PCM reset cycle involves leaving the battery unplugged for at least 5 minutes though I like to , plugging it back in and taking it on a drive of 10 miles. This clears any adaptive conditions the ecu may have made from sensor feedback. It doesn’t always help especially if it is a problem with say the throttle body itself, but this at least sets back to baseline values I believe.

 

[ActuallyaMazda]

And my idle issue has been potentially solved! I went ahead and switched out the MAF with a new one and as soon as I fired it up after resetting the PCM, it held a normal idle and warm up cycle. Drove it home from work with no issues, parked it and it idled back to where it normally sits for park. I had tried cleaning the old one with MAF cleaner, but it might have just been faulty in some way even after a few months of having it in there. It was a Transpro MAF and I replaced with a Duralast. Will update if the problem returns, but for at least for now, it doesn’t seem to have the same issue.

 

[ActuallyaMazda]

Do your engines have EGR valves? What if they are opening and then not shutting correctly? That would cause a vacuum leak.

It’s a Federal emissions model, so no EGR on this one.

 

[ActuallyaMazda]

Update: High idle is completely gone after a day of driving. I have a low choppy idle of about 400 - 500 rpm on startup now until it finds a little bit of temperature and then after that it seemingly operates normally. Removing FPR vacuum line evens idle out to about 800 - 900 while it is chopping like it has a massive cam in it. Switching out FPR back to old one to see if that makes a difference after work today. It really does make a huge difference having an OEM Motorcraft IAC on there as well as everyone else has stated in other threads even if it is one that has been cleaned from the junkyard. Idle held a lot more stable with less surging with the Motorcraft one rather than one from Standard or another 3rd party brand.

 

[superj]

At least it seems you have made a huge amount of headway. Check temp sensors, or senders, whichever one tells the ecu the truck trmp so it can adjust fuel till the o2 reaches operating temp and pulls the truck out of loop mode.

It can be called a sender or a sensor so check them all if you have two or three. Your current description can be a temp sensor or maybe spark plugs gapped wrong,.wpuld be my starting points

 

[ActuallyaMazda]

At least it seems you have made a huge amount of headway. Check temp sensors, or senders, whichever one tells the ecu the truck trmp so it can adjust fuel till the o2 reaches operating temp and pulls the truck out of loop mode.

It can be called a sender or a sensor so check them all if you have two or three. Your current description can be a temp sensor or maybe spark plugs gapped wrong,.wpuld be my starting points

Haven’t replaced my ECT yet, but couldn’t hurt to. I’ll also check that spark plug gap tomorrow as well.

 

[gaz]

@ActuallyaMazda ,
- The 94 Mazda 4.0l has two (2) O² sensors, one just aft of each exhaust manifold on the connecting exhaust tubing.
- Though the 93 and 94 Ford 4.0l versions have an EGR valve on the California version, the Mazdas and Aerostars and Federal versions will not.

• Did you replace the throttle body gasket with a new gasket when you reinstalled it?
• If you removed the throttle position sensor (TPS) you may not have it in exactly the correct spot.
• There is at least one vacuum line hooked the throttle body, perhaps it is loose or cracked after remove and install?

 

[ActuallyaMazda]

• @ActuallyaMazda ,
  - The 94 Mazda 4.0l has two (2) O² sensors, one just aft of each exhaust manifold on the connecting exhaust tubing.
  - Though the 93 and 94 Ford 4.0l versions have an EGR valve on the California version, the Mazdas and Aerostars and Federal versions will not.
  
  • Did you replace the throttle body gasket with a new gasket when you reinstalled it?
  • If you removed the throttle position sensor (TPS) you may not have it in exactly the correct spot.
  • There is at least one vacuum line hooked the throttle body, perhaps it is loose or cracked after remove and install?

  Thanks for the response! The gasket has been changed out with the throttle body, both O2 sensors are now in and operational, and the vacuum line coming off the Throttle body is capped off since the previous owner yanked out all the emissions equipment attached to that. The TPS could be a concern, but from the symptoms I’m having now, I don’t think it’s likely. It’s seeming like it wants to run rich until it gets almost up to temp and then once it hits temperature, the RPM’s drop like I unplugged the IAC, even with it still being plugged in.

 

[ActuallyaMazda]

I replaced my ECT sensor tonight along with checking the gap on my spark plugs. One spark plug was gapped way too small, so I got that set right with all of them at .052 and with the new sensor plugged in and all KAM erased, I plugged the battery in and fired it up. Everything was great. It idled like a dream, drove like a champ and got up to temp without failure. Took it for a test drive for another 10 miles and all went great. On my way home, I stopped by the gas station because I was getting pretty low after all the idling and running rich that has happened this week, and after I got done at the pump, I got ready to leave and the truck cranked and cranked but no start. It wasn’t getting any fuel from what it seemed, so I popped over to the EEC connector and jumped the fuel pump to the negative post to try and get it pumping and with it jumped to the battery, the truck fired right over and started to run up in RPMs really quick so I unhooked the jumper wire and left it and to my surprise and absolute dismay, the high idle was back. With the engine completely warmed up after the drive it sat at the gas station holding at 1300 rpm. I put it in gear to leave and it jumped forward a bit when it dropped down to 800 rpm. I got it back home and put it back into park and it idled back to 1000. Shut it off and waited 10 seconds before trying to turn it back on again and when it fired back up it jumped to 3000 for a few seconds, came down to 2000 and then settled down at 1300. I think I can at least take it work tomorrow and I’ll tinker with it more once I get it on a lift and can pull the bed off to check fuel pump wiring.

 

[superj]

Oh man, what a bummer. I thought you were going to say all is well.

I hope you find something easy like a broken vacuum line or something and its a quick easy fix.

 

[ActuallyaMazda]

Oh man, what a bummer. I thought you were going to say all is well.

I hope you find something easy like a broken vacuum line or something and its a quick easy fix.

I can’t say with confidence that all is well, but I am happy to report that I drove it to and from work today with absolutely zero issues. I did nothing to it at all today except plug the battery in after I unplugged it last night to clear memory again. I didn’t notice anything but a slight miss the first time I came to a stoplight, but after that it ran like a charm the whole way home with taking the long way to make sure it properly gets up to operating temperature. Putting it in park it went up to about 1000 rpm. I haven’t driven it since, but it’s had plenty of time to cool down completely so it’s a fresh cold start when I got start it next. Hopefully the next thing I can post on here is that it hasn’t given me any further issues after that little hiccup at the gas station.