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lebolewis · 08-06-2006, 01:40 PM

I am always AMAZED at the breadth of knowledge of the readers on this site.

So, here is a Tow Vehicle question for you.

I just purchased a 94 GMC Suburban with a 454 engine and TBI injection. I am now learning that it idles a bit too fast. When I stop at a traffic light, I have to hold pressure on the brakes to keep from going forward. If I release the brakes, it will creep forward pretty fast. I have looked everywhere for an idle adjustment and there is NONE! And...it hasn't escaped my attention that the fast idle burns a lot of GAS! I purchased a Haynes Repair Manual, but for idle adjustment, it simply says "take it to the dealer!" A lot of help that is!

Any suggestions on what to check or what to do would be appreciated. Bill

overlander63 · 08-06-2006, 01:59 PM

Does the check engine light come on while it is idling? You may have an IAC that is blocked partly open.
The IAC, or Idle Air Control motor, is located on the right rear of the throttle body assembly. It should take about a 15/16" wrench to lossen it, and it unsrews easily. Clean out the hole (inside the throttle body) and the V shaped plunger on the motor. Install it, and see if the idle goes down.
If the check engine light is on, you can get clues of what is going on from them, although what the code says, is not always what the problem is.

lebolewis · 08-06-2006, 03:35 PM

Quote:

Originally Posted by overlander63

Does the check engine light come on while it is idling? You may have an IAC that is blocked partly open.
The IAC, or Idle Air Control motor, is located on the right rear of the throttle body assembly. It should take about a 15/16" wrench to lossen it, and it unsrews easily. Clean out the hole (inside the throttle body) and the V shaped plunger on the motor. Install it, and see if the idle goes down.
If the check engine light is on, you can get clues of what is going on from them, although what the code says, is not always what the problem is.

No, the "check engine" light is not on. I will remove the IAC and clean it and re-install it to see if that helps. I'll let you know. Thanks, Bill

overlander63 · 08-06-2006, 04:14 PM

Something else it could be, is simply a broken vacuum line. It will also make the engine rev up a bit, so if the cleaning (which won't hurt anything) doesn't make a difference, check for cracked or broken vacuum lines.

davidz71 · 08-06-2006, 05:41 PM

My son's '91 C1500 started idling faster and we found that it was a brake booster going bad. This goes right along with the vacuum leak suggestion.

Pat370 · 08-06-2006, 06:07 PM

Spray WD40 around the base of the TBI where the TBI meets the hot water pod and also where the hot water pod meets the intake manafold. This a common problm with the TBI on the 454 engines. If the gaskets are bad and leaking vacuum the engine idle will slow down when the WD40 enters the hole in the gasket. If this is the case then you will need to replace the gaskets. Also check the same way where the manafold gaskets meet the heads. You do have a vacuum leak.

Good luck,
Let me know the results
Pat370

davidz71 · 08-06-2006, 06:15 PM

Also disconnect the rubber hose going to the brake booster. Put your finger over the hose and if the idle slows down to normal then it is the diaphram in the brake booster.

john hd · 08-06-2006, 07:22 PM

check the base gasket on the tbi unit.

a quick check is to take an unlit propane torch (gas on) and hold it around the base of the tbi unit.

if it revs up you have found the problem.

john

Mike Lewis · 08-06-2006, 10:20 PM

Quote:

Originally Posted by Pat370

Spray WD40 around the base of the TBI where the TBI meets the hot water pod and also where the hot water pod meets the intake manafold. This a common problm with the TBI on the 454 engines. If the gaskets are bad and leaking vacuum the engine idle will slow down when the WD40 enters the hole in the gasket. If this is the case then you will need to replace the gaskets. Also check the same way where the manafold gaskets meet the heads. You do have a vacuum leak.

Good luck,
Let me know the results
Pat370

Throttle body base gaskets are a common problem, above is a simple check for that----Pieman

lebolewis · 08-07-2006, 11:16 AM

Thanks for the ideas - Craig, Pat370 and John. I will try the WD40 next. I just got a new can of that. Unfortunately, I sold my propane torch in a garage sale a few years ago, thinking that I would never need it again!

Now I wish I had it!

Terry- I removed and cleaned the IAC valve (pintle) and the valve seat and re-installed it and that did not fix the problem. However, I have some questions about what is normal...or NOT about this valve. I expected the pintle to freely move in and out, since it has a spring for tension.......but it does not! It takes a great deal of pressure to make it move in or out. And when it does move, it moves very slowly and feels like it meets a great amount of friction. Then it occured to me that this was a MOTOR and the pintle rod, through some kind of a gear reduction mechanism was connected to this motor and when you tried to push the pintle in and out you had to overcome the gear reduction to turn the motor. If that's true, then maybe this great resistance to movement was normal? Can you clarify that? If this is not normal, then I've got a bad IAC valve. I also measured the resistance between the connection pins A-B and C-D and got 54 ohms on each pair. The Haynes Manual says that is normal.

I'll wait to hear something before I try the WD-40. Bill

Oh, I forgot to mention that at idle, the RPMs on my Tach is 1100 RPMs with the Trans in neutral and 1000 with the Trans in Drive. Don't know how accurate the Tach is.

john hd · 08-07-2006, 11:26 AM

give it a shot of WD can't hurt!

i had the same exact problem on a late 80's chevy. when i pulled the tbi body off there was literally nothing left of the gasket! huge air leak!

john

Pat370 · 08-07-2006, 11:46 AM

When the IAC valve is bad the engine will die when your foot is removed from the gas pedel. I am sure that you have a bad TBI base gasket.

Let me know how you come out,

Pat370

Mike Lewis · 08-07-2006, 04:24 PM

If the gasket is bad enough you can look down through the throat of the throttle body and see that it's out of place---pieman

overlander63 · 08-07-2006, 04:43 PM

Quote:

Originally Posted by Pat370

When the IAC valve is bad the engine will die when your foot is removed from the gas pedel. I am sure that you have a bad TBI base gasket.

Let me know how you come out,

Pat370

Exactly. If it was gunked up, it could not close enough to control idle, now that it is clean, he can check for a vacuum leak, and be fairly certain it is not (almost) the only electronic control part that is causing the problem.

lebolewis · 08-07-2006, 05:32 PM

OK, one more question before I tackle the TBI gasket. The Haynes Manual shows a top view of the TBI with the 2 injecters removed. I don't plan on removing my injectors. There are 3 bolts, roughly in a triangle, that hold the TBI in place. However, my TBI does not look EXACTLY like the one pictured in the Haynes Manual. My air filter, that mounts on top of the TBI, has 2 rods about 4 inches apart with wing nuts that hold it on. These rods go through the filter housing and are attached to the TBI in about the same location that 2 of the TBI mounting bolts are supposed to be. The bottom of the rod bases are formed like a bolt head and look like they could be bolts that screw out. I haven't tried them, but if these are not 2 of the TBI mounting bolts, then I am stumpt!! Comments please. Bill

overlander63 · 08-07-2006, 06:15 PM

Bill, your TBI mounting bolts/nuts should be at each corner of the assembly, for a total of 4. The injectors should each have a pinch-clip plug that holds the wires on them. Unsnap the throttle cable, unscrew the two fuel lines (supply and return), unplug the throttle position sensor and IAC, and unscrew the four mounting bolts. The unit should come up off the intake manifold.

scottanlily · 08-07-2006, 09:49 PM

Hello lebolewis ,

you are correct in your assumption on the IAC .It is an idle air control motor.
It is controlled by the computer (PCM) and does not move freely and do not
try it with the IAC out and plugged in to the wire harness ,turning on the key
as the pintle will be commanded to close by the PCM and the pintle will come right out ,you will need a new IAC.Vacuum leak is most likely the cause as
has been stated here and any of the above can do it .This is where a scan
tool comes in handy ,but it should be easy to find otherwise.

Scott

lebolewis · 08-08-2006, 07:37 AM

OK guys, this is TBI gasket day! I will first do the testing with the WD40 and see what happens. If I get no indications of a vacuum leak (engine slows in RPM), then I will see if I can buy/borrow a Propane Torch and test with that (engine speeds up). If I do detect a leak, then I've got to see if my local Auto Parts places have the correct replacement gasket, before I remove the TBI. Don't want to be stranded with no truck and no gasket!

To be continued......... Bill

Mike Lewis · 08-08-2006, 08:52 AM

It won't be necessary to check it both ways. If the gasket it the problem you'll know with either test. ---Pieman

lebolewis · 08-08-2006, 09:00 AM

Quote:

Originally Posted by scottanlily

Hello lebolewis ,

you are correct in your assumption on the IAC .It is an idle air control motor.
It is controlled by the computer (PCM) and does not move freely and do not
try it with the IAC out and plugged in to the wire harness ,turning on the key
as the pintle will be commanded to close by the PCM and the pintle will come right out ,you will need a new IAC.Vacuum leak is most likely the cause as
has been stated here and any of the above can do it .This is where a scan
tool comes in handy ,but it should be easy to find otherwise.

Scott

Scott, what is a "scan tool"? Bill