Intermittent Rough Running Diagnosis

**sturgillbd** · 10-14-2019

I knew it wasnt $0D . Thats one reason I asked. Ill look through and see if I can find anything in the datalog that stands out. To log knock retard on some of the masks, you have to make a bin change so it shows up in the datastream. I'll see what it takes on yours in a bit.

**tony-baroni** · 10-14-2019

Thanks Brian!

I'm not sure where to go next.
Here's what I did:
1. cleared codes
2. Started engine, pulled vacuum lines one by one of the TBI to see what effect it would have. Some of them changed the idle slightly, and then I put my finger over the port on the TBI, and it would act just like as if the hose was in place. Other hoses, did not change the throttle when I unplugged them, and left them open.
The big difference came when I unplugged the vacuum hose that comes from the rear of the TBI, and plugs into a sensor (MAP?) beside the TBI. When I unplug that, the idle drops considerably, from the sound of it, near a normal level.
3. I didn't understand your IAC advice. The screw I think you're talking about is the one that sets the position of the throttle blades when the throttle is closed, right? On my engine, it is already backed out all the way. I backed it out about a 1/4 turn before the screw pulls away off the throttle bracket, because the blades are already touching the sides of the bores in the TBI.

How do I set the IAC now?

I wonder if there might be a break in my base gasket under the TBI....I've traced all the vacuum lines I can find twice now, and they all appear plugged in where they should be.

**sturgillbd** · 10-14-2019

Check your base gasket on the throttle body. If no vacuum leaks, Adjust the minimum air screw to close the throttle blades for approximately 10 IAC counts at hot idle. Clear PCM codes. You have constant knock retard. You could possibly have a bad knock sensor or bad filter board on the memcal. Do you hear any spark knock/ping? You can simulate a knock sensor on this pcm with a 3.9k resistor. It thinks a knock sensor is there but gets no knock feedback. You have extremely low timing everywhere in the datalog and that is why it is lacking in power. I also see the timing all over the place with a constant throttle/load. Something is making the PCM pull timing. As always, make sure all of the engine and PCM grounds are in good shape. Make sure plug wires aren't inducing noise back into the PCM through the engine harness. Get your fault codes cleared up and not returning then datalog again posting results here.

**sturgillbd** · 10-14-2019

If base gasket is good, you could have an intake gasket leak. Make sure the pcv valve is free and if it has vacuum power brakes, make sure you don't have a leak there. It appears you have a vacuum leak somewhere if you have backed the minimum air screw all the way out with no change.

**mecanicus** · 10-14-2019

this helped when I had to set my IAC :

**tony-baroni** · 10-17-2019

Terrific guide, thank you mecanius!

**tony-baroni** · 10-17-2019

Finding the root cause continues to elude me...let me share what I've done in a couple checks this evening and last.

Replaced the TBI base gasket, torqued to specs 12 ft lbs.
Verified the hose to the MAP holds vacuum
Verified the hose and vacuum canister hold pressure (though they slowly leak, not enough to make this idle so high)
Verified the PCV valve and hose don't have any cracks, and is in place
Verified the hose from the TBI to the EGR control unit is in place.
Verified the hose from the EGR to the EGR control unit holds vacuum

Found that the charcoal canister hose, that runs from a port on the front of the TBI, does NOT hold vacuum. I do not know if it is supposed to!

Then I tried setting the IAC.
Warmed engine up and in closed loop mode.
Turned key off, jumped the 1 & 2 terminals on the ALDL port, put key to ON for ten seconds. I have read the IAC should be buzzing at this point, but there was no noise. Unplugged IAC. Key off, jumper out. Plugged in PC, and started vehicle, IAC showed zero, and idle was still 1900. Is it possible my IAC is not working and holding it wide open? Any other checks I should try?

Pressed on the underside of the EGR valve, which dropped the idle while I held the valve open. So that doesn't appear to be a leak...

I also tried using Wd40 spray on the ports and intake gaskets but it didn't seem to have any effect. I then tried spraying it directly into the TBI, and it didn't have any effect there either. The fan is so strong at 2000 rpms that it's just dispersing the wd40 too quickly.