‘94 won’t pass emissions but NO CEL

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VC-120 should be fine, not sure what else to try, it is puzzling for sure.
 
I found the timing was definitely advanced, probably around 5-6*. After I put white paint on the timing mark, it was much clearer where the timing was at, so I set it to 3* and noticed a slight/minuscule drop in idle speed so I will drive it around today and possibly take it for an emissions test if the idle stays consistent. When all this started, the CEL was intermittent and would sometimes come on with elevation changes but the CEL would always turn off on its own as soon as I got back to my “home elevation”. Now the CEL is forever on (originally a code 25/26 combo but got the code 26 to go away) and when I clear it, at the instance it lights up again, there is a noticeable change in idle speed and it comes on only at idle like when I pull off the highway at an off ramp light. When it comes on the RPM’s jump up as if a switch was turned on -like if you turned on the A/C or something. I’m just hoping it’s not a valve issue.
 
So I was looking at the exhaust manifolds because after my last smoke test I noticed a small amount of smoke in the area after I started the vehicle -my eyes were trained to finding smoke after the hour of smoke testing I guess. I hooked up the smoke machine to the exhaust as WC recommended and sure enough smoke was coming from the manifold. I removed the heat shields and found that a couple of the manifold nuts were loose on the first 3 cylinders so I went on the rear manifold and found the stud was free spinning… The stud appears to have stripped out the seat where it resided.
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When you smoke test the engine. You need to cap off the intake where it goes into the snorkel. You have a gaping hole where smoke is coming out of that isn't going to let it build pressure and leak out. I suggest smoke testing from right behind the mass air flow sensor.
 
The stud is no problem, you can helicoil it.

The exhaust manifold can actually draw in air, hard to believe but it can.

When smoke testing pull off block or rocker cover breather to see if smoke is getting into the innards of the engine, unlikely but I have seen it from internal cracks in the cylinder head.

Again rare, but I must say this thread is beginning to look 'iffy'.

Regards

Dave
 
Thanks all… Look, I’m not very experienced (but willing to learn) nor do I claim to know what I’m doing, so I’m not sure what’s “iffy” other than my lack of experience, lol. I’m truly only seeking advice on an evolving issue because clearly something unexpected (to me) is going on here since now finding out that the manifold’s seal is compromised. Thanks for the direction towards the helicoil because truly that’s all I’m looking for is help and I’ve never used one so it seemed like a bad situation initially.
The lean condition “activating itself intermittently” (probably with increasing heat) makes complete sense with what’s been going on in my instance and all tests per the sequence of the manual are checking out as “normal”, so with my obvious inexperience hopefully someone else who’s possibly encountering the same issues finds help in my search for the solution. This has been evolving from no/intermittent CEL (after clearing several other codes successfully) to chasing a code 25 that I couldn’t track down, so I’m hopeful that I found the resolution. Thanks again.
 
I did the helicoil and it worked great and held strong so the manifold is now sealed but I’m STILL getting a lean code so I switched the oxygen sensors between the PAIR valve manifold ports 1 & 2 (again, these are new sensors BTW) because I read in another thread that the 93/94 NTK o2 (24080 and 24044) sensors were specific for the 3 cylinders. After switching the sensors, the idle was spot on. I went for a drive and it seemed a tad sluggish before 2000 rpm’s then it suddenly began to stumble at idle and almost stalled at the light and then started being really sluggish and then the CEL lit up with a code 26 so now it is running rich. I used a stethoscope to listen to whether all injectors were firing and all 6 were ticking. I’m getting a vacuum tester/gauge because the smoke test checked out fine and I will test the VAF again, but I’m at a loss and may have to take it to the dealership depending on what I find.
 
Could you have a intermittent short or some other problem (electrical) in your O2 sensor wires?
 
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109madrid said:
I did the helicoil and it worked great and held strong so the manifold is now sealed but I’m STILL getting a lean code so I switched the oxygen sensors between the PAIR valve manifold ports 1 & 2 (again, these are new sensors BTW) because I read in another thread that the 93/94 NTK o2 (24080 and 24044) sensors were specific for the 3 cylinders. After switching the sensors, the idle was spot on. I went for a drive and it seemed a tad sluggish before 2000 rpm’s then it suddenly began to stumble at idle and almost stalled at the light and then started being really sluggish and then the CEL lit up with a code 26 so now it is running rich. I used a stethoscope to listen to whether all injectors were firing and all 6 were ticking. I’m getting a vacuum tester/gauge because the smoke test checked out fine and I will test the VAF again, but I’m at a loss and may have to take it to the dealership depending on what I find.
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Have you found a cause of your problem or taken it to a dealership? I'm having the same code 25 problem and I've been working on it for a long time. Any information will be appreciated.
 
No, I haven’t worked on it lately nor taken it to the dealership. Sorry I don’t have anything good to share but I recently bought a vacuum gauge and fuel pressure gauge so I will be running some tests there and report back. I did check the o2 sensor wiring and they were fine and I also adjusted my VAF and that cleared the CEL for a month or so and then the 25/26 CEL combo came back so I put the VAF adjustments back to stock and I’m still getting code 25.
 
I found a problem that may have been throwing the error code 25. It was the O2 sensor heater voltage (HT2) that was not generated in the ECM. The HT2 voltage is around 14.2 V while engine is running and it's 12V with just the ignition ON. I was getting 12V with ignition ON and only 5V with the engine running. I was able to find this because the bank 1 (HT1) was working properly. After fixing the ECM (a bad soldering on the 5W resistor) I began to see the multimeter needle flipping and I stopped seeing the CEL comes on while driving.
 
masyam said:
I found a problem that may have been throwing the error code 25. It was the O2 sensor heater voltage (HT2) that was not generated in the ECM. The HT2 voltage is around 14.2 V while engine is running and it's 12V with just the ignition ON. I was getting 12V with ignition ON and only 5V with the engine running. I was able to find this because the bank 1 (HT1) was working properly. After fixing the ECM (a bad soldering on the 5W resistor) I began to see the multimeter needle flipping and I stopped seeing the CEL comes on while driving.
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Seeing as the 93 and 94 trucks are both OBD1 they can use the same ECM/engine computer ( 95 to 97 are OBD2 and their ECM won't work). Problem is, the 93 trucks had a problem with their ECM, so Toyota issued a SB to replace the older 93 ECM's with an updated part number used on the 94 trucks. With such a limited supply of these "one year only" ECM's being available* this is all the more reason why it's a good idea to buy an extra ECM from a 94 that's being parted out so you have a spare. *I just checked on line stock with Napa, Auto Zone, O'Reilly, and Pep Boys, NO ONE is showing any stock on the 93/94 ECM'.
 
I made some progress here finally and my fuel pressure and compression tests rendered no issues, but when I tested the engine’s vacuum it wasn’t registering on the gauge at all even after all the focus on the vacuum lines. I didn’t think of these items previously but I noticed oil leaks from the oil cap and distributor after cleaning up the engine bay so I replaced the oil cap O-ring and found that the distributor o-ring was not OEM; it was very small and brittle, and pretty much non existent because it wasn’t even making contact anymore. I also replaced the grommet on the brake booster’s vacuum line. So at this point the idle is correct and smooth, there is no rattle upon startup (it also had a slight wine like the belts or pulleys were squealing or something of the sort and that’s totally gone now), nor the feeling of misfiring, and it’s been running great, however I’m still getting an intermittent code 25 that will go away if I rev the engine up around 2500 rpm’s for a few seconds. I’m happy with the progress though and I replaced the battery terminals with mil spec terminals to rule out ground issues. At this point I am going to check the NSS and possibly readjust the TPS because I didn’t apply vacuum (I simply pinched the hose) during my last TPS adjustment and go from there.
 
It took me 3 times to pass California smog test after eliminating the code 25. It was failing at idle speed. Readjusting the TPS improved the numbers, but not good enough to pass the test. Replacing the VAF took care of the problem and it passed the test with very good numbers. I purchased a new VAF on Amazon for $120. It gives me a slight hesitation when start moving from a complete stop, which I did not have with the original VAF.
 
What I meant by easy emissions is that they don’t really inspect things visually (my missing PAIR valve passes visual without a second look) and thankfully no dyno checks here either but none of that matters if I can’t get this thing to pass.

Thanks for the input all, that info on the VAF is especially promising. It’s frustrating that essential parts for the 93/94’s like the PAIR valve, ECU, and the VAF are pretty much unobtainium -I guess I’ll just buy a jeep and waste money fixing a new vehicle that breaks down regularly instead of my 20+ year old money pit:flipoff2:
 
Solved: The EBay/Amazon VAF eliminated the CEL and the lean condition however it still had a significant intermittent miss that would periodically pop up at a red light or randomly when idling otherwise it ran fine especially when the RPM’s were up past idling… So I checked the compression and started to get vastly varying readings on my #5 cylinder -all other cylinders were reading 190PSI consistently but #5 was as high as 180 but as low as 40PSI when tested randomly over about a month’s period of time. I had done a compression check multiple times before and never got below 189 on any cylinder so I was thinking it was the autozone tester, so I borrowed another and the same thing happened. I finally bit the bullet and took it to a shop and told them about the whole intermittent code 25 and paid for a diagnostic. They also pinpointed the miss to cylinder 5 and said they got 60PSI and refused to work on it because they said it would be in the shop too long with the current machine shop turnaround time being at around a month, according to the owner of the shop. They also couldn’t reproduce the conditions for the CEL so I’m hoping that’s in the past for good. I am going to do a leak down test but the previous test was fine, but I was told by the shop that it wasn’t the head gasket because air definitely wasn’t leaking into the coolant so I’m thinking I have a bad valve on the #5 cylinder or something along those lines. We will see with the test. I’m not sure what to do at this point because there aren’t any shops in my area that will work on a ‘94 and this is probably a bigger job than I can handle at the moment.
 
109madrid said:
Solved: The EBay/Amazon VAF eliminated the CEL and the lean condition however it still had a significant intermittent miss that would periodically pop up at a red light or randomly when idling otherwise it ran fine especially when the RPM’s were up past idling… So I checked the compression and started to get vastly varying readings on my #5 cylinder -all other cylinders were reading 190PSI consistently but #5 was as high as 180 but as low as 40PSI when tested randomly over about a month’s period of time. I had done a compression check multiple times before and never got below 189 on any cylinder so I was thinking it was the autozone tester, so I borrowed another and the same thing happened. I finally bit the bullet and took it to a shop and told them about the whole intermittent code 25 and paid for a diagnostic. They also pinpointed the miss to cylinder 5 and said they got 60PSI and refused to work on it because they said it would be in the shop too long with the current machine shop turnaround time being at around a month, according to the owner of the shop. They also couldn’t reproduce the conditions for the CEL so I’m hoping that’s in the past for good. I am going to do a leak down test but the previous test was fine, but I was told by the shop that it wasn’t the head gasket because air definitely wasn’t leaking into the coolant so I’m thinking I have a bad valve on the #5 cylinder or something along those lines. We will see with the test. I’m not sure what to do at this point because there aren’t any shops in my area that will work on a ‘94 and this is probably a bigger job than I can handle at the moment.
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I agree w/ @Dave 2000 .

You most likely have a valve sticking. It could be from debris, carbon buildup, or a bent valve. Either way, you're looking at doing a head gasket / head rebuild.

You can try dumping all sorts of products in the fuel or whatever and running it to see if there is an improvement, but you may only delay the inevitable.

How often do you drive this and get it good and heated up for extended periods of time?

Change the oil and take it for a good long two hour sustained drive at 75 MPH.
 
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