rough idle / loss of performance (1 Viewer)

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ok...so this is a strange one that I have been working on for a while now. Gonna throw the background in here because it is either something I touched, or a great coincidence.

first: '91 FJ80 ~297k

~3 months ago I had overheating issues. Ended up replacing the thermostat and while doing that found that multiple electric sensors in the thermo housing were brittle and cracked/disintegrated upon removal. 2 of them are no longer made by toyota and could not find any after market, so ended up buying used ones on the interwebs. Got all things put back together and did not have any overheating issues anymore.

After taking it out for a few local test drives I found that I had some vacuum leaks as well. So replaced all vacuum lines since most of them were super dry and cracking.

New issue suddenly appeared after those first few drives...when I initially start the truck after sitting overnight, it runs fine...no issues. Once I get to operating temp, then shut the truck off and let sit for a few minutes (like long enough to get gas) and then start it again, I have a rough idle and loss of power. This only happens if I shut the truck off and then turn it on again while at operating temp. If I start the truck in the morning...and drive/run it all day (tested this over a weekend) it will run fine.

I took it in to have it diagnosed today and the mechanic told me that it needs a valve adjustment...while I do not disagree with that, I think that there is more than that going on with some sensor or something. I have no idea how to test or where to go from here. I have done some searches and have seen things about vacuum (replaced all of them) and EGR valve (they dont make those anymore, and I have NOT tried to clean mine yet) It just really feel like it is something computer related due to when the issue presents itself...or it is super coincidental and the work I did has nothing to do with it.

Also it is throwing codes, 25 and 26...rich and lean. So I did replace the O2 sensors with brand new OEM and new gaskets (but the nuts were really worn, maybe bad seal on them?)

any help is appreciated...I need to get back out.
 
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I'd re-check the vacuum hoses that you replaced and make sure they are all still properly seated and that none of them popped off or that one didn't get missed. I don't have a 3fe so not too helpful for you but just a thought since it was something you recently replaced. Maybe check the air intake for leaks as well?
 
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A few thoughts in no particular order.
3FE valve adjust every 15K miles according to the FSM. If yours hasn't been touched in 297K miles, it may be time to consider it.
Rough idle is almost always caused by unmetered air entering the system. The top half of this motor wants to be sealed. That includes silly things like the gasket on the oil fill cap, the gasket on the oil dipstick, the grommets on the valve cover, the large intake plenum, etc.
However, since you replaced the coolant temp sensors on the thermostat housing and you mentioned that the wiring was suspect, and you replaced the O2 sensors, I would certainly look into that as the ECU uses all that feedback for fuel trim.
Remember that this is essentially a fuel injected tractor motor from 1986. ALWAYS look for the simple things first. When was the last time she had a good old tune up? Are those plug wires 30 years old?
 
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I'd re-check the vacuum hoses that you replaced and make sure they are all still properly seated and that none of them popped off or that one didn't get missed. I don't have a 3fe so not too helpful for you but just a thought since it was something you recently replaced. Maybe check the air intake for leaks as well?
100% agree and I did check those...like 4 times...hahaha.
 
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A few thoughts in no particular order.
3FE valve adjust every 15K miles according to the FSM. If yours hasn't been touched in 297K miles, it may be time to consider it.
Rough idle is almost always caused by unmetered air entering the system. The top half of this motor wants to be sealed. That includes silly things like the gasket on the oil fill cap, the gasket on the oil dipstick, the grommets on the valve cover, the large intake plenum, etc.
However, since you replaced the coolant temp sensors on the thermostat housing and you mentioned that the wiring was suspect, and you replaced the O2 sensors, I would certainly look into that as the ECU uses all that feedback for fuel trim.
Remember that this is essentially a fuel injected tractor motor from 1986. ALWAYS look for the simple things first. When was the last time she had a good old tune up? Are those plug wires 30 years old?
I for sure think the valves need to be adjusted, and agree with the unmetered air...but why would it run fine after initial start and keep running fin as long as it is not shut down? That is the part that has me stumped. I will go back and check out all those seals though...It should be good but worth taking another look for sure.
As for tune up...she is due, but I dont think that would cause the issue that I am having...wires, plugs, cap and coil are all on the list. Though the pugs and wires have been replaced in the last 100k.
 
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Yup, I did clear the codes and they did return. I was going to pull the battery cable and reset it again.
Simply removing the 15 amp EFI fuse in the fuse block on the left fender for 5 or 10 minutes is a less invasive way of resetting the ECU. No need to play with battery terminals.
It is also good practice to do this whenever a sensor is changed, or even after a basic tune up. It allows the ECU to relearn all sensors and clears out any stored error codes.

If the codes come back, then you have an issue that you need to address.

As for why the engine is behaving the way it is, until you address the basic mechanicals and basic maintenance, we are all just speculating. Don't overcomplicate a tractor.
 
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Joined
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Simply removing the 15 amp EFI fuse in the fuse block on the left fender for 5 or 10 minutes is a less invasive way of resetting the ECU. No need to play with battery terminals.
It is also good practice to do this whenever a sensor is changed, or even after a basic tune up. It allows the ECU to relearn all sensors and clears out any stored error codes.

If the codes come back, then you have an issue that you need to address.

As for why the engine is behaving the way it is, until you address the basic mechanicals and basic maintenance, we are all just speculating. Don't overcomplicate a tractor.
Great tip on the ECU reset...I will use that going forward and while I agree some of the basic maintenance needs to get done...none of that would explain the root issue of rough idle only after it is shut off and turned back on...valves or plugs would act in a consistent manner. I am trying to track down the issue before adding more variables.
 
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Also since you haven't mentioned, check your air intake hose...mine had a hairline crack and caused the car to sputter at low rpm yet would go away once warmed up- but would return , say after a fuel up- I assumed the heat expanded the material to seal it enough to not leak. Duct taped the piss out of it a la field fix and replaced with a silicone one a few days later. Zero issues since.
 
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Also since you haven't mentioned, check your air intake hose...mine had a hairline crack and caused the car to sputter at low rpm yet would go away once warmed up- but would return , say after a fuel up- I assumed the heat expanded the material to seal it enough to not leak. Duct taped the piss out of it a la field fix and replaced with a silicone one a few days later. Zero issues since.
ah good call on that...mine was trashed and I replaced it prior to this issue...it is essentially brand new. Both parts.
 

bentonrover

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Don't discount that coolant temperature sensor. Problem started after over heating? When they go bad they mess up the air/fuel ratio. Meter it or just buy from rocksuto. I had a similar issue with my 1990 fj62. Rig went crazy after ripping out the rear heating after the lines rusted out.
 

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