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Discussion Starter · #1 ·
1991 K5 TBI 350 with SM465. One out of five times the idle will seem to fall out when taking it out of gear and coming to a stop. It feels like a misfire. It will correct itself after about 5 seconds, can usually correct it a little quicker by revving. Less commonly, it will do this going into first gear from a stop and I'll need to give it gas to come out of it. It never actually dies. It doesn't matter if it's hot or cold. I have rebuilt the throttle body with new injectors, replaced IAC and MAPS, and repaired several vacuum leaks in lines. Any other suggestions?
 

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Discussion Starter · #3 ·
Yes, i followed a process to reset the IAC from a YouTube video that hopefully was correct. That whole process with jumping A-B, set idle screw, disconnect and reconnect battery, etc. Is there more to calibration? I'll check pressure and injector spray tonight. Thanks for the ideas!
 

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‘01 Silverado 2500HD 8.1/Allison 5sp xcab long bed
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Help me understand something, please? Idle screw?
 

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Discussion Starter · #5 ·
Help me understand something, please? Idle screw?
There's an idle adjustment screw on the front driver side of the throttle body housing behind a steel plug. You need to punch a hole in the plug to pull it out, then you have access to the idle set screw. You need access to this when you reset the IAC. My understanding is you don't need to reinstall the plug after adjustment.
 

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That's what I suspected. You only touch this if you've ever messed with the actual throttle plate. This should not need to be adjusted. That's why it's behind a cover. That screw sets the throttle plate dead stop. The IAC is under the plate and allows a set amount off air in behind it as the computer requires.


If your idle was at around 650 without the IAC connected, then it should be fine.

I've never had IAC issues that required uncovering the plate set screw. Others may have due to wear issues. It's just rare that you have to touch it.

As Peter said, check your fuel pressure. Can't begin to tell you how many issues are directly related to that. Also, when was the last time your changed your coolant temp sensor by the thermostat?
 
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If your idle was at around 650 without the IAC connected, then it should be fine.
I had to adjust the min stop screw after my build, one thing though, be careful if you do adjust it. If you unscrew it too far the throttle cable will extend to the point that the sleeve over it will catch on it and cause a hard spot in the pedal.

Is there more to calibration?
Yes, you should check your TPS voltage too, open the attachment I posted above and read through it to confirm your U Tube video was the same for the IAC and further down it shows how to calibrate the TPS.
 

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Discussion Starter · #8 ·
If I remember right I think I turned the screw a quarter turn, maybe half at the most. Last night I was sent in a completely different direction when I opened the hood, had a radiator leak to attend to. Tonight I'm going to swap for a new coolant sensor, test the TPS, and check fuel pressure. I'll get back to you tomorrow.
 

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Discussion Starter · #9 ·
I replaced the coolant sensor last night and finished up my radiator swap. Tried a fuel pressure test kit from O'reillys, the actron fitting leaked like hell. Need to find another kit to test fuel pressure, then test the TPS tonight.
 

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I don't know how much you want to invest in it but here's a fairly cheap system for monitoring and adjusting your fuel pressure.
Scroll down a little to see the adapter, gauge, and regulator.

Here's what it looks like, you'll have to bend the MAP vacuum tube a little and drill an offset hole through the plate but, it's pretty simple. Make sure you use plenty of Teflon tape or it'll leak!
IMG_0833.JPG


Here's what it looks like mounted and running
AFPR.JPG
 

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If you need to use Teflon tape, don't use the white, it's for water and will resolve in gas.
 

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I don't know, I've used this stuff on lots of different applications along with PTFE (Teflon) paste on others. In this application I ran it for over 6 months until I changed the 28PSI spring to an 18PSI one set @ 15PSI to run 7.4 (80LB/HR) injectors, took it all apart, re-taped the fittings, and put it back together without any issues. The tape came off in the same condition as it went on. Those injectors didn't work very well (wouldn't idle well) so I switched it back to the 5.7L (61LB/Hr) injectors and set it to 20PSI which I'm running now. As far as I know Teflon is Teflon. I think the washers for the fuel line fittings are Teflon too. I may be wrong but it's worked for me so far.
 

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I used white on a furance oil line once and had a leak,I found out the hard way.

Whiten or silver Teflon is for small water pipes
Red/pink is for large water pipes, 2" and up
Yellow/orange is for fuel lines, natural gas, furance oil and gasoline
Green is for air lines, i.e.: oxygen
 

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Discussion Starter · #14 ·
I found a pressure test kit that doesn't leak, I have a very steady 11psi on the outlet side of a new fuel filter. My TPS test shows .536v at rest, 4.523 WOT and what appears to be a brief dead spot somewhere around 4v, but I can't pin down exactly where. I just get a very quick OL when I get around 4. I'm going to watch the injector spray pattern once I put everything back together.
 

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Whiten or silver Teflon is for small water pipes
Red/pink is for large water pipes, 2" and up
Yellow/orange is for fuel lines, natural gas, furance oil and gasoline
Green is for air lines, i.e.: oxygen
Well, you learn something new every day, I didn't know they had those different colors/compounds.

My TPS test shows .536v at rest, 4.523 WOT and what appears to be a brief dead spot somewhere around 4v, but I can't pin down exactly where. I just get a very quick OL when I get around 4.
I'd replace it.
 

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Discussion Starter · #16 ·
Yep. I have an ACdelco coming. About time to replace spark plugs too. I'll get back to you guys next week.
 

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Discussion Starter · #17 ·
It seems replacing the ACdelco TPS and spark plugs (ACdelco CR43TS at .035 gap) either didn't do anything or possibly made it worse? Now when the stumbling starts there is no getting out of it. If I do manage to get it started and idling, touching the throttle at all will make it lose idle all together. While I was stumbling around checking things, I noticed there is no vacuum at the J and A ports on the front of the throttle body, at least none at idle. This can't be right, can it?
 

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Discussion Starter · #18 ·
Let me say first that I'm not a fan of modifications as a solution to a problem, but in this case I'm just gonna roll with it. I bought a EGR block plate at O'Reillys. I had to modify the holes to mount it up and capped the vacuum port on the throttle body. I took the blazer for a 45 minute test drive in town and on highway and hadn't had a single hiccup. It runs better at all speeds and all rpm as far as I can tell. I verified the valve diaphragm wasn't leaking by depressing by hand and then holding the vacuum port and it held, and the gasket was intact so I'm guessing either it was sticking open or the controller is bad. I'm strongly considering leaving the plate on and calling it good.
 

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Discussion Starter · #19 ·
I began getting 32 code (egr) every time I drove 20 highway miles and the sputtering issue when hot returned. I bought a new ACdelco egr valve and new vacuum line from it to the egr controller. I don't get the check engine light or 32 code anymore but still have some sputtering /acceleration issues. Diag continues...
 
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