93 Suburban - Idle Only Problem

snsime · August 15, 2007

I am so happy at being able to have gotten assistance here for my 03, thought I might see if there's any advice on a nagging problem with our "beater".

5.7l, has been well maintained and up to two months ago ran like a top. We had it at the shop to have the oil pressure sending unit replaced, as it was starting to fail.

When we got it back, the truck would intermittently stumble at idle, then catch itself. Sometimes it was virtually all the time, hot or cold, sometimes it was very infrequent, on one drive it worked fine. It got progressively worse. At its worse, the idle would be almost constantly fluctuating between virtually stalling and revving up again to compensate.

I checked all wires, leads and connections -and for codes - there was nothing. I finally noticed that the IAC motor was very loose in the TBI. I took it out, and replaced it with another IAC motor I had from last year (left over from when I was throwing parts chasing a problem when we first bought it )

This made a significant difference, in fact for a short while, I thought it solved the problem.

However, it still exists, but is definitely less pronounced. It is definitely an issue ONLY at idle.

There are still no codes.

Should I perhaps replace with a new IAC motor, or is there anything else we should consider checking?

Thanks!

-Scott

snsime · August 16, 2007

OK, not a very interesting topic I guess... so maybe I'll run down what I've done, and if there's anything else I should be trying, or elsewhere I should be looking, perhaps someone will share

I found the IAC initially very loose in the TBI. I found it loose because I had dug out a used IAC that I had in my toolbox. So, instead of simpy tightening, I replaced it. In retrospect, I wish I would have tried tightening first.

Anyway, the idle problem is greatly diminished, but not gone.. still intermitent, but not nearly as pronounced.

For next steps, should I try disconnecting the IAC and see what happens? That should give me base idle (I think), and if the problem persists, can I rule out IAC? Is there anything I should check with the TPS? EGR diaphram can be moved by hand, and it has vaccum. There are no codes.

Any help would by very appreciated.

Thanks

-Scott

bobcat5544 · August 16, 2007

Sorry I ment to reply to you yesterday.

The IAC usally would not cause the truck to Idle ruff hot or cold just at idle.

Unpluging the IAC Wont give your a base idle.

Try this if you have a propane torch or a can of starter fluid.

Whit the engine running turn the torch on don't light it or spray starter fuild around the intake and vaccum hoses and see if the eng rpms raise up. If they do the you have a vaccum leak. (make sure you dont allow it to get sucked in though the thottle body)

You may have a small vac leak. that engine is known for vac leaks.

snsime · August 16, 2007

Thanks very much, I'll give that a go tonight.

I should try and describe the symptom better though... the idle isn't rough, so to speak. It will idle glass smooth, interupted with varying frequency by a pronounced drop in RPM, it will then surge back to recover. Sometimes it will do this a little, sometimes a lot.

Before I replaced the loose IAC, it had gotten so bad the truck would stall sometimes at idle. Since I replaced, both frequency and sevearity of these idle interuptions have dimminished signigicantly, however it still exists.

As I type this, I begin to wonder if it might be electrical (coil, module)... although I was thinking it must be realted to the idle circuit as that's the only time there's any noticable problem.

bobcat5544 · August 16, 2007

Checking for vac leaks gives me a base to start at. when there is a vac leak the computer will use the iac to compensate for a vac leak, which will can cause the up/down rpms

snsime · August 16, 2007

Hello,

OK. I took the IAC out and cleaned it in carb cleaner - looked clean anyways.

Did a visual of all vacuum hoses and connections, everything seemed OK.

Started it, ran fine, no idle disruption. Took it for a fairly long start-and-go jaunt.

After about 20mins, it seemed fairly close to operating temperature. I hadn't stopped long at any stop signs, but it seemed symptom free.

Pulled behind garage, let it idle. After about 5 seconds it got it's first fart in the idle. Then a few more, and a few more. Park or in gear, no difference. Very incosistent in terms of sevarity and frequency.

I sprayed starter fluid all around vacuum hoses, connections, TBI, intake, intake to heads. If there's a vacuum leak, I sure can't find it.

Disconnected brake booster vacuum and plugged inlet on TBI, no change. EGR, same action and no change, charcoal canister, no change. I then disconnected what I believe is the MAP sensor (thin rectangular black plastic module on the right bank close to IAC, with a vacuum line into back of TBI. After I plugged the TBI inlet... it seemed to stumble and hunt, then it went to a slighty higher idle and stayed nice and steady there, no more farting. Plug vacuum line back in, and idle came down, and after about 5 seconds would start the idle interruptions again. I repeated this maybe 4 times, same result each time.

I disconnected the harness from the MAP (if that's what it is) and it almost immediately stalled. Plugged back in harness, restarted truck (noticing that I know had a code, from disconnecting the MAP I would guess), ran fine for maybe 15-20 seconds, then idle interruptions started again.

That's about all I could find. Any thoughts? Could it be the MAP?

Thanks,

-Scott

snsime · August 17, 2007

A little more info...

I let the truck cool for maybe 3 hours, and went back out again. By this time a friend had dropped by with his 94 K1500. We pulled off his MAP and were going to see if we could get the idle problem to resolve on a swap.

Starting it cold, like earlier today... it ran fine at first. I let it idle for 20 min, no problem. Took it for a ride around the block... nothing. Longer drive, still nothing. After a third drive, maybe 15 minutes today "in gear" time, I let it sit in drive, foot on brake. Quite a while goes by, and then the inevitable fart. Then after maybe 30 seconds, another (by fart, I mean a very noticable drop in RPM...it seems like it's going to stall but catches at the last moment and returns to idle speed). This goes on for maybe 5 minutes, it farts maybe 6 times. I put it in park, and then it starts to fart more regularily. I get out of the truck, pop the hood, and it farts bad enough to stall out. I swap the MAPs, restart, and it runs fine for maybe 30 seconds, and then starts to fart again. Turn it off, swap my MAP back on, start it up, runs fine for another 30 seconds, then starts to fart again.

I am far from a technician (obviously) but I find it difficult to imagine this could be a vacuum leak.. it's too intermitent and seems to be waylaid temporarily by cycling ignition on and off, or when it's colder.

Sigh. Any thoughts at all?

Thanks

ChevyTech007 · August 17, 2007

I suspect either clogged passages in the throttle body to the MAP and PCV ports, and or a cracked magnet on the distributor shaft.

snsime · August 17, 2007

Thanks, I think the passages are OK, I put a vacuum gauge on just about every port on the TBI this AM - except the brake booster. They all seem to be pulling fine.

I'll pull the distributer tonight.

This all started very suddenly. My oil pressure sending unit stopped working 2 months ago, and at the time it was at an exhaust shop, so they offered to replace the unit. The moment we got the truck back it had this problem. I know where the sending unit is, and I'm wondering if they moved or removed the distributor to replace it.

I'm also wondering if this new sending unit could be causing the problem? I'll have to look closer, but it seems as though it's got a wire running into the control module on the distributor...

Grrrr.

Rob94F250 · August 17, 2007

I have a 95 Suburban with the same TBI engine, and had a similar problem.

The gasket between the intake manifold and TBI sometime crack causing a vac leak - replaced mine on spec.

The other thing that may cause it is the EGR valve. You can try disconnecting and plugging the vacuum line to the EGR and see if the problem goes away. If it fixes it, the EGR is gone.

Rob.

ChevyTech007 · August 18, 2007

I think the passages are OK, I put a vacuum gauge on just about every port on the TBI this AM - except the brake booster. They all seem to be pulling fine.

Be careful here, a MAP port can be almost all the way clogged and still show good vacuum but it will cause the reading to be "lazy" and cause problems. I have seen it more than once. Just 2 cents worth.

Spdfrkss · August 24, 2007

I think the passages are OK, I put a vacuum gauge on just about every port on the TBI this AM - except the brake booster. They all seem to be pulling fine.

Be careful here, a MAP port can be almost all the way clogged and still show good vacuum but it will cause the reading to be "lazy" and cause problems. I have seen it more than once. Just 2 cents worth.

not to hijack this thread but iam having the same problem,, Mine is a 91 gmc k1500 305 spd, I pulled the tbi and found that the pcv port was completely clogged , so i cleaned all the ports after taking apart the tbi. put all new gaskets and fliters and springs in the tbi and installed it back onto the truck. Now it seems that the idle problem went away, but now when i drive the truck, it seems to lose power when i accelerate, also when i am crusing in 4 th or 5th gear and i remove my foot off the throttle it starts to buck, so i either have to accelerate or apply the clutch to get it to stop.. Now i have changed all the plugs, wires, cap, rotor, ign module, tps,temp sender( both on top and side), oil pressure sender, knock sensor, fuel filter, air filter,thermostat, checked the egr,and egr soleniod, checked for vacumm leaks, Cleaned the iac, and adjusted curb idle, check timing ( with timing connector disconnected) and I still have the problem.. Does anyone have a clue as what could be the problem.. Because i am lost as what else to check for.

Spdfrkss · August 28, 2007

I think the passages are OK, I put a vacuum gauge on just about every port on the TBI this AM - except the brake booster. They all seem to be pulling fine.

Be careful here, a MAP port can be almost all the way clogged and still show good vacuum but it will cause the reading to be "lazy" and cause problems. I have seen it more than once. Just 2 cents worth.

not to hijack this thread but iam having the same problem,, Mine is a 91 gmc k1500 305 spd, I pulled the tbi and found that the pcv port was completely clogged , so i cleaned all the ports after taking apart the tbi. put all new gaskets and fliters and springs in the tbi and installed it back onto the truck. Now it seems that the idle problem went away, but now when i drive the truck, it seems to lose power when i accelerate, also when i am crusing in 4 th or 5th gear and i remove my foot off the throttle it starts to buck, so i either have to accelerate or apply the clutch to get it to stop.. Now i have changed all the plugs, wires, cap, rotor, ign module, tps,temp sender( both on top and side), oil pressure sender, knock sensor, fuel filter, air filter,thermostat, checked the egr,and egr soleniod, checked for vacumm leaks, Cleaned the iac, and adjusted curb idle, check timing ( with timing connector disconnected) and I still have the problem.. Does anyone have a clue as what could be the problem.. Because i am lost as what else to check for.

anyone have any clues.. or did this topic stump even the GM Techs? It is funny how no one can get a straight answer to these so called mysterious problems...

randykio · February 1, 2009

what a bunch of bullcrap ! i see the same problem i have , posted several times on this site , but still no solutions ! wtf ! surely someone has fixed this problem , but no fking reposts to help anyone out ! what is this some kind of fking gay truck chatroom ? i want to know what the solution is - was ? for christ sakes !