**SOLVED** Codes 24 & 31 on 1994 FZJ

FMC80

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Dudes,

Here's the background: My truck is running pretty great. Solid idle and acceleration, no hesitation. This may or may not be related to the main issue but on very few occasions, my idle will wander and fluctuate approx 200 rpm up and down the range. A little throttle and the problem goes away.

The main issue: On three separate occasions over a 2 month period, my truck died for no discernable reason.
1st time, 2 months ago: Rolling with light throttle, approx 15 mph and I saw the CEL illuminate then the truck died. I started it immediately and it drove just fine thereafter. No CEL and no codes after jumping the diag port.

2nd and 3rd time happened consecutively today. After about a 20 minute drive, I pulled over and was idling. I saw the CEL illuminate and then the truck died. I started it back up and it died again. After looking around the engine bay pretending to know what the hell I was looking at, I started the truck and it drove fine for the rest of the entire day.

When I turn the ignition to "on" I have the CEL. I installed new fusible links about a month ago as PM.

After getting home, I jumped the diag port and got 31 "Volume Air Flow Signal". I checked the FSM and did basic ohm outs on the VAF and all checked within limits. After reinstalling the VAF harness, I checked the diag port again and this time I have a 31 and a 24 "Air intake temp sensor signal" fault.
Screen Shot 2021-07-14 at 9.57.29 PM.png


Now, I never messed with my VAF except to clean it and put some dielectric grease on the connection. I never messed with screws or anything of that sort.

Aside from the diag port telling me I have codes 24 and 31, my truck drives fine and the CEL is not illuminated at all.

My truck has a new fuel filter, the main engine harness was inspected and gtg, EGR is disabled, all new plugs, wires, distributor, timed to 3*.

Finally, aside from checking for continuity, and removing and reinstalling the EFI fuse, what else can I do? I've searched through the archives and maybe my google-fu sucks so I'm looking for any assistance you can provide.

Cheers,
Pell
 
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FMC80

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Took the VAF off today and bench checked it per FSM. Resistance is within limits.

I found a couple of threads that show similar issues so I’ll be studying those and see if it leads me anywhere. I’m hoping it isn’t my main engine harness but who knows.

 
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FMC80

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Been trying to dup this issue. The few times it happened on a slight uphill with low RPMs. Specifically on my driveway and few mild hills on base. All times less than 25MPH. Code 24 went away and never returned but code 31 remains.

I'm going to slowly go back over my work from my baselining project and try to retrace steps.

I was thinking this may be a fuel-related issue at first since it happens sometimes on low/constant RPM and usually on low uphill grades.

I need to recheck my timing as well.

After checking the VAF/AFM I noticed oil in the air intake hose and throttle body. See pics.
IMG_1469.jpeg
IMG_1470.jpeg

Additionally, I checked all my plugs:
IMG_1471.jpeg
IMG_1472.jpeg

I think I'm running a bit hot which is why I want to recheck my timing. What are your thoughts on these plugs?

Finally, I did a compression test just knock it out for a data point. All cylinders are between 140 and 150.
 
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How old is your rubber intake tube? These degrade over time and will leak air through the bellows flex joint, as this air enters downstream of the VAF it is unmetered and will cause problems. You can wrap the flex joint in duck tape and see if that improves anything, if so its time for a new intake tube.
 

FMC80

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How old is your rubber intake tube? These degrade over time and will leak air through the bellows flex joint, as this air enters downstream of the VAF it is unmetered and will cause problems. You can wrap the flex joint in duck tape and see if that improves anything, if so its time for a new intake tube.
Thanks for the response. Not sure how old it is but I’ve inspected it numerous times and can’t visually detect any cracks/splitting.
 
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The intake tube can definitely fail with no visible signs. My 94 was running rough at idle to the point it would die at stop signs/lights and the intake tube looked perfectly fine. I wrapped some tape around the flex joint and it completely fixed the issue, then ordered a new tube as a long term fix.

If your intake tube is original its now 25+ years old and likely not in great shape. It takes 5 minutes to tape up the flex joint and see if thats the issue.
 

FMC80

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The intake tube can definitely fail with no visible signs. My 94 was running rough at idle to the point it would die at stop signs/lights and the intake tube looked perfectly fine. I wrapped some tape around the flex joint and it completely fixed the issue, then ordered a new tube as a long term fix.

If your intake tube is original its now 25+ years old and likely not in great shape. It takes 5 minutes to tape up the flex joint and see if thats the issue.
Nothing to lose here, I’ll give it a shot. Thanks man
 

FMC80

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Update: Aaaand the Code 24 is back. Title updated to reflect.
Used plastic and lots of tape to seal the air intake tube and drove it for a few days and it didn’t fix the issue. Dont know how old the intake tube is but it is in good condition. I've inspected it thoroughly but I may replace just for SnGs.
Yesterday, I drove the cruiser and the CEL came on and idle stumbled. The CEL extinguished by itself while driving. It came on and off 2x. It seems to happen at low speed/rpm and otherwise the cruiser drives smooth and accelerates well.
I’m still tripping code 31.

Also, I keep getting oil inside the air intake tube and my throttle body before the valve.
I checked my new OEM PCV And it seems to be fine.

I went through and tried to check all my vacuum lines and didn’t find anything awry.
Would appreciate any insight you may have.
Thanks,
Pell
 
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FMC80

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Okay, just to baseline, looks like the troubleshooting per the FSM is pointing to a ECM or VAF. I checked the VAF on the truck and on the bench and it is gtg so maybe it's the ECM? I just gotta figure out how to do it now.
 

FMC80

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Update again:

Troubleshooting code 24:
Screen Shot 2021-07-24 at 7.03.32 PM.png

Shot between the ECM and VAF, gtg.
Screen Shot 2021-07-24 at 7.03.57 PM.png


Then Shot the ECM to ground:
Screen Shot 2021-07-24 at 7.04.32 PM.png

Other than checking the main engine harness for breaks, what do you guys think?

ECM? VAF? I replaced the fusible links again just in case. Maybe that'll fix everything, hahaha.
 

Irish Reiver

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I think your focus should be to address the oil in the throttle body first. I have never seen that before but can't imagine it helping the situation. That amount of oil (along with what you can't see) is being pulled through the TB and intake and is likely clogging everything up with carbon. Ports on the TB will probably have been blocked preventing vacuum being applied to places that need it. This could be messing with your fueling.
I would suggest the following:-
1) Pull the valve cover and clean it thoroughly - especially under the baffles. Oil must be settling on the baffles for the PCV to vacuum it up like that. (Gasket P/N 11213-66021)
2) Replace the PCV & hose and the breather hose. (PVC Valve 12204-35040, PCV Grommet 90480-18001, PCV Hose 12261-66021, Breather hose 12262-66021
3) Pull the TB and clean it - make sure you get all the little orifices. (Throttle body gasket 22271-66010)
4) Optional- Pull the upper intake chamber and clean that also.
5) Optional - Fit an oil catch can.

Also worth checking the vacuum lines since your in there anyway.

The FSM test procedure eventually leads you to "Replace ECU" but to my knowledge they rarely go bad. In any case I think I have one for a 94 if needed.
 

FMC80

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I think your focus should be to address the oil in the throttle body first. I have never seen that before but can't imagine it helping the situation. That amount of oil (along with what you can't see) is being pulled through the TB and intake and is likely clogging everything up with carbon. Ports on the TB will probably have been blocked preventing vacuum being applied to places that need it. This could be messing with your fueling.
I would suggest the following:-
1) Pull the valve cover and clean it thoroughly - especially under the baffles. Oil must be settling on the baffles for the PCV to vacuum it up like that. (Gasket P/N 11213-66021)
2) Replace the PCV & hose and the breather hose. (PVC Valve 12204-35040, PCV Grommet 90480-18001, PCV Hose 12261-66021, Breather hose 12262-66021
3) Pull the TB and clean it - make sure you get all the little orifices. (Throttle body gasket 22271-66010)
4) Optional- Pull the upper intake chamber and clean that also.
5) Optional - Fit an oil catch can.

Also worth checking the vacuum lines since your in there anyway.

The FSM test procedure eventually leads you to "Replace ECU" but to my knowledge they rarely go bad. In any case I think I have one for a 94 if needed.
Thanks for the response.
For background I already replaced every single thing you stated in your post last month. I replaced everything with OEM components and I already ordered all the consumables for the VC in case I need to rip into it again.

What you suggest does make sense though, there must be something going on with my valve cover baffles or something for all that oil to collect. I might just get a new VC and get this one professionally cleaned.

ABC90063-E86B-456B-9FA7-140ECBDC9E15.jpeg
5C5D93B1-0E9B-4AD4-A3D5-F6B7350241F4.jpeg
2630B378-8E8C-4C38-9255-F45602E8FC36.jpeg
05ED1E36-CF74-49C0-B9F4-41933C2128F5.jpeg
When I had it off a month ago, it had some deposits and buildup but I spent several days soaking it with brake cleaner and carb cleaner and simple green. Water seemed to flow through the baffles without any obstruction.
Maybe I knocked something loose and it is obstructing proper oil flow now

Again, thanks a lot for you response. Gives me more push to break all back apart again.
 

FMC80

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Update: Ordered new valve cover and all consumables. I think the original VC baffles are definitely obstructed based on research I found through Mud. When this happens, I think the VC vent tube is sucking the oil out and up into the throttle body and intake tube. I'm hoping that the excess oil in the intake is causal to the 24/31 codes.
 
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Looking at your spark plugs, they look like they are running lean with the white soot covering them or possible contamination. I would check your timing to make sure it isn't too far advanced.

As for the code 24, if the new rubber boot doesn't fix the air leak, then check to make sure that the flap in the AFM is free from obstruction. The fact that the code 24 is intermittent sounds like an air leak in the intake plenum. A smear of Vasaline can be used to seal the rubber intake boot on both ends and keep out air and dirt. I'm baffled as to how the oil is getting into the intake maybe a vaccuum situation pulling it into the intake, but maybe your new parts can fix that.

If everything checks out, try running another AFM from a known working donor vehicle and see if it was going bad. They are getting very hard to find these days so it's a good idea to buy a used spare if you can find one. Good luck!
 

FMC80

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Looking at your spark plugs, they look like they are running lean with the white soot covering them or possible contamination. I would check your timing to make sure it isn't too far advanced.

As for the code 24, if the new rubber boot doesn't fix the air leak, then check to make sure that the flap in the AFM is free from obstruction. The fact that the code 24 is intermittent sounds like an air leak in the intake plenum. A smear of Vasaline can be used to seal the rubber intake boot on both ends and keep out air and dirt. I'm baffled as to how the oil is getting into the intake maybe a vaccuum situation pulling it into the intake, but maybe your new parts can fix that.

If everything checks out, try running another AFM from a known working donor vehicle and see if it was going bad. They are getting very hard to find these days so it's a good idea to buy a used spare if you can find one. Good luck!
Hey brother, good looking out. I agree about the spark plugs, when I checked them I noticed they weren't exactly 14lbs so upon reinstall, I made sure and hit that torque setting. Also checked timing and it is dead on balls accurate at 3*. I'll be pulling the plugs in about 100 miles for analysis.

We'll see about the intake tube, I just bought another one so once it comes in I'll report. I appreciate the Vaseline input, I have plenty of that :flipoff2:. All new clamps and gaskets going on (again).

VAF/AFM door is gtg, no obstructions. Thanks for the overall insight. I know I'll fix this....I'm just gonna get the parts cannon out and spray and pray.

Pell
 

FMC80

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Still waiting on parts to arrive so I can break into my truck. The problem is very intermittent which makes me think it is related to the oil buildup in my intake tube and throttle body. There’s been no code for the last 3 weeks but I am still cleaning out my intake tube and throttle body of oil every few days.
The trucks runs fine but every three weeks or about 200-300 miles the engine will sputter, die, and the CEL will pop. The truck starts right up and drives as if nothing happened.
Once my parts get here I’m going to hit tank the my VC and intake plenum. Hopefully that remedies this issue.
 
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FMC80

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Got some parts in today. I’ve installed the new intake tube first because it’s low hanging fruit. As @sogafarm stated, it may be this simple. The new boot is much like my wife; soft and supple and the old one is much like my mother in law; old and brittle.
23A43464-625C-48E6-933D-E217D8D47C78.jpeg


53F0705D-54D3-453A-96A0-316594C11BB8.jpeg
52465AB8-86BF-4E7C-AE48-F713C577122B.jpeg


Interesting note on my new VC; the baffles are different than the original that is currently installed. Reference the oil passageways in the new vs old. The tab is completely removed on the new one and the overall design is different.
B559EF72-1250-4A43-A157-F15EF95A963F.jpeg
B780335E-379B-468C-A241-B826D305523D.jpeg
 
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FMC80

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Still awaiting parts. But one interesting note/observation: I swapped out the new intake tube and cleared all codes. Drove it off and on for about 4 days and today the CEL came on. I pulled the code and it was a 24 but no 31. Once I clear codes and drive, sometimes it takes weeks for it to trip the CEL and it will more often than not pull a code 31. Other times, it'll trip both 24 and 31. Today it only tripped a 24 which was a first.

Not sure if anyone who is proficient in troubleshooting is reading this but if you are, what do you make of that?

I still think it has something to do with my VC baffles, I'm hoping anyway. Once I'm able to swap it out and drive it, we will see.
 
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Code 24 is the cold air sensor. It's triggered by the AFM. As mentioned before, if you can get a hold of another AFM to switch out with yours to see if it's a bad AFM, you could maybe rule that out or it could be the problem. Somewhere in the FSM there is a testing procedure for it, but if it's a code that comes on and then off randomly then it might not register in a stationary test. The good news is your getting closer to a fix.
 

FMC80

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Code 24 is the cold air sensor. It's triggered by the AFM. As mentioned before, if you can get a hold of another AFM to switch out with yours to see if it's a bad AFM, you could maybe rule that out or it could be the problem. Somewhere in the FSM there is a testing procedure for it, but if it's a code that comes on and then off randomly then it might not register in a stationary test. The good news is your getting closer to a fix.

Good points again, thank you for your insight. I think I’m in denial. I hope it’s not the VAF/AFM but I suppose it isn’t that bad if it is.
As of this morning all parts have shipped so getting closer to doing some maintenance.
 

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