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Discussion Starter · #1 ·
Looking for some help, I have a 99 that stumbles and backfires when coming off idle or tip in throttle.
Through data logs sent off my tuner was able to identify the IAT2 temps were way to hot. I replaced the IAT2 sensor, made sure the I/C pump is constantly running, and also replaced the wiring harness and PCM. I have checked the harness next to the headers and all harnesses running into the ECU. What am I missing that would cause the IAT2 to cycle from 2*- 256* then it reads -256* until it reaches 0* then cycles back to positive temps. Vacuum reads 12-14 at idle built truck ported everything.
So far, this is what I have replaced or attempted in correcting the issue.
-new fuel pumps, jumped regulator, new hi/lo relays, had 60lb injectors flow tested, fuel pressure is perfect.
- new coil packs
- new intake gaskets, rtv intake from KB intake.
- new 02 sensors
Any help or suggestions would be appreciated, Truck has been down for almost 2 years with this issue.
Truck is fully built KB 2.6 HC truck

Thanks in advance, also no BS if you are able to help me get this thing back on the road I will gladly compensate your time. :bigtu
 

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Previously got my wire colors messed up - should see around 5 volts on the red/yellow wire with the sensor unplugged and the key on and the grey/red wire should show a short to ground - if the grey wire doesn't show a short to ground (zero resistance), IAT-2 readings will 'float' around. The grey/red wire is the signal return (ground) line to the PCM - might be broken in the plug harness.


Bird
 

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I've personally had bad grounds really screw with air temp sensors on other things so that's where I would start good luck
 

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What do the plugs look like?
 

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Looking for some help, I have a 99 that stumbles and backfires when coming off idle or tip in throttle.
Through data logs sent off my tuner was able to identify the IAT2 temps were way to hot. I replaced the IAT2 sensor, made sure the I/C pump is constantly running, and also replaced the wiring harness and PCM. I have checked the harness next to the headers and all harnesses running into the ECU. What am I missing that would cause the IAT2 to cycle from 2*- 256* then it reads -256* until it reaches 0* then cycles back to positive temps. Vacuum reads 12-14 at idle built truck ported everything.
So far, this is what I have replaced or attempted in correcting the issue.
-new fuel pumps, jumped regulator, new hi/lo relays, had 60lb injectors flow tested, fuel pressure is perfect.
- new coil packs
- new intake gaskets, rtv intake from KB intake.
- new 02 sensors
Any help or suggestions would be appreciated, Truck has been down for almost 2 years with this issue.
Truck is fully built KB 2.6 HC truck

Thanks in advance, also no BS if you are able to help me get this thing back on the road I will gladly compensate your time. :bigtu
ill fix it for you with a short answer. cams are to big.

long answer...... the cams are to large and the overlap to much causing reversion. increased temps like this are a result from exhaust gases being pulled back into the engine. hope this helps...
 

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Discussion Starter · #10 ·
Thanks for all the responses,
Bird I will get those voltages for you soon, as far as tuning it I completely trust who is tuning it and would rather wait until I have crossed all my possible mechanical issues before I say it's in the tune.
Plugs look perfect, but I don't know how much of a reading you can get off of a few short blasts. I can pull them again and get photos if everyone thinks that is the issue. I'm running tr6's gap at .42 roughly 17lbs of boost.

As far as cam overlap that's the first I have heard about that, sure hope that isn't it. Can't say I haven't thought about a stuck valve letting exhaust back up into the intake. Thought I would see more residue in the lower intake if that was the case.

I am located in Tulsa Oklahoma, I have considered shipping the truck off but it's just too expensive for my taste.
 

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Gap the plugs at .32 -.34
 

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Thanks for all the responses,
Bird I will get those voltages for you soon, as far as tuning it I completely trust who is tuning it and would rather wait until I have crossed all my possible mechanical issues before I say it's in the tune.
Plugs look perfect, but I don't know how much of a reading you can get off of a few short blasts. I can pull them again and get photos if everyone thinks that is the issue. I'm running tr6's gap at .42 roughly 17lbs of boost.

As far as cam overlap that's the first I have heard about that, sure hope that isn't it. Can't say I haven't thought about a stuck valve letting exhaust back up into the intake. Thought I would see more residue in the lower intake if that was the case.

I am located in Tulsa Oklahoma, I have considered shipping the truck off but it's just too expensive for my taste.
A tune wouldn't cause erratic iat2. It's mechanical/electrical. Like said, that plug gap is way to big.
 

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Would a clogged intercooler cause this? Restriction of air through the intercooler? When was the last time you cleaned it?
 

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What intake setup? JLP inlet stuff or stock KB? What Maf? What size tube.

Cams were mentioned earlier, but no cam specs were detailed here. What cams/heads?

I see Eric Korn(MBBS) is trying to help. That is a real good start! Pay attention.



Jim
 

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That's not a accurate IAT2 reading, -256 and +256 are the lowest and highest the PCM can infer. It's either the wrong PID selected or the firmware for the hand held needs updated etc.

This is another good point - I always use 'Downstream Air Temps' when datalogging - anything else gives me garbage readings, even with the latest firmware.

Bird
 

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Lots of questions about mechanical stuff, but based on the readings your getting, im with bird and eric, its either a bad sensor, bad PID selected or some wiring. I wouldn't sweat anything else until you figure out why its reading incorrectly
 

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Looking for some help, I have a 99 that stumbles and backfires when coming off idle or tip in throttle.
Through data logs sent off my tuner was able to identify the IAT2 temps were way to hot. I replaced the IAT2 sensor, made sure the I/C pump is constantly running, and also replaced the wiring harness and PCM. I have checked the harness next to the headers and all harnesses running into the ECU. What am I missing that would cause the IAT2 to cycle from 2*- 256* then it reads -256* until it reaches 0* then cycles back to positive temps. Vacuum reads 12-14 at idle built truck ported everything.
So far, this is what I have replaced or attempted in correcting the issue.
-new fuel pumps, jumped regulator, new hi/lo relays, had 60lb injectors flow tested, fuel pressure is perfect.
- new coil packs
- new intake gaskets, rtv intake from KB intake.
- new 02 sensors
Any help or suggestions would be appreciated, Truck has been down for almost 2 years with this issue.
Truck is fully built KB 2.6 HC truck

Thanks in advance, also no BS if you are able to help me get this thing back on the road I will gladly compensate your time. :bigtu
Vacuum reads 12-14 at idle
Truck is fully built KB 2.6 HC truck
You might have the wrong bypass actuator. Do you know for sure its the low vacuum one and if so can you verify its actually working and adjusted correctly?

JJ
 
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