1990 Chevy Truck Idle Problem

JENSPORT · 06-26-2008, 09:22 AM

I Have another trivia for you guys. My truck will not go slow. If you give it the tiniest amount of gas at low speeds it wants to die. If it starts to die and you leave off the gas it will idle it self back up. On the interstate or driving normal it's fine. Here is what I have checked or changed. The IAC has been changed. The throttle body has been changed. Complete tune-up. It is coding map sensor but that has also been switched. Any ideas.

dcraker · 07-02-2008, 08:34 AM

have you checked over your oxygen sensor?

ColinRyan2 · 07-02-2008, 08:36 AM

what size motor is in it?

ColinRyan2 · 07-02-2008, 08:40 AM

Vaccum leak could be causing problem. It would also cause a code for Map sensor voltage to high.

JENSPORT · 07-02-2008, 08:43 AM

Okay thanks for the input. Here are your answers the motor is a 5.7 350. The oxygen sensor has already been changed. I have looked around for Vaccum leaks but can't find any is there a spot that you can think I might want to focus on. This truck is driving me nuts. I call it Christine because things just seem to happen and we can't find an expanation.

dcraker · 07-02-2008, 08:51 AM

does it still do this on a cold start and before you actually have to drive anywhere?
dual exhaust with dual catalytic converters? or a single catalylic converter?

if you have one, then maybe you could try to disconnect it in front of the 'cat, then run it and see if it still does this, if it runs great (loud of course) then it could be your 'cat.

OR, bang your 'cat converter some, to knock some of the debris loose, and run it and see if that changes the pitch any

ColinRyan2 · 07-02-2008, 08:53 AM

You can focus around the intake manifold. around the throttle body and for any cracks in the vaccum lines running to the firewall or brake booster. Its sometimes hard to see or hear a vacumm leak. I my self will sometimes use a small spray of carb cleaner around any area were a vaccum leak may be. When you spray carb cleaner on a vaccum leak the engine ruff idle will sometimes clear up or cause the engine to run worse. it is normally noticeable if that carb cleaner is being sucked into the engine through a vaccum leak. BUT BE VERY CAREFULL WHERE YOUR SPRAYING. CARB CLEANER IS VERY FLAMMABLE!!!! DONT USE TO MUCH. JUST A LIGHT SPRAY.

One other thing you can look at is the engine coolant temp sensor. Chevy small blocks and 4.3 liters are bad for running ruff when the sensor is reading wrong. Need to make sure what the engine temp sensor says is the actuall temp of the engine.

JENSPORT · 07-02-2008, 11:36 AM

The truck will start right up and idle smooth for a couple minutes then you will hear it start to miss. The main problem is when it is gear and you are driving slow like out my driveway or in traffic. It will start to spit and sputter if you touch the gas peddle slightly. If you try like to hold the brake and give a little gas so it won't stall it works the opposite then it will definitely stall. If you let off the gas completely it will idle back up but still has a miss. The catalylic converter is hollowed out. I will try spraying the cleaner to find a leak. I already also tried the coolant sensor it has been changed.

ColinRyan2 · 07-03-2008, 07:07 AM

have you checked to see if the egr valve is stuck open. It will cause a rough idle at low rpms. would cause a stall when you first hit the accelerator. there should be no vaccum at idle going to the egr valve. if there is could be the egr valve solenoid. If there is no vaccum, while the engine is cool see if the plunger inside the egr valve is functioning. On that model you should be able to move the plunger inside the egr valve from the small opening on the outside. its should move up and down and you may hear a sucking sound because its a vaccum diaphram that lifts the plunger.

JENSPORT · 07-15-2008, 12:43 PM

Well guys thanks for your input but I believe I finally figured it out. Knock on wood. The vacuum line from the egr valve was on the wrong thing on the map sensor. I feel like an idiot. All that aggrevation for 1 simple little thing. Thanks again for your help.

**Midnight Tech** · 07-15-2008, 05:56 PM

Quote:

Originally Posted by JENSPORT

I feel like an idiot.

Please don't call yourself that - we all have things that throw us off like that.

BUDFAN8 · 07-17-2008, 07:55 PM

hey i've searched for hours more than once because of a simple wire not being connected. i get in a rush ( mostly because people think stuff should be done yesterday) and it ends up taking longer.

Bud.

06-26-2008, 09:22 AM	#1 (permalink)
JENSPORT Registered User Join Date: Aug 2007 Posts: 9 OS: win xp	1990 Chevy Truck Idle Problem I Have another trivia for you guys. My truck will not go slow. If you give it the tiniest amount of gas at low speeds it wants to die. If it starts to die and you leave off the gas it will idle it self back up. On the interstate or driving normal it's fine. Here is what I have checked or changed. The IAC has been changed. The throttle body has been changed. Complete tune-up. It is coding map sensor but that has also been switched. Any ideas.

07-02-2008, 08:34 AM	#2 (permalink)
dcraker Registered User Join Date: Jan 2008 Posts: 62 OS: Windows 2000	Re: 1990 Chevy Truck Idle Problem have you checked over your oxygen sensor? __________________ Thanks, David If you cover it in chocolate, it'll look better.

07-02-2008, 08:36 AM	#3 (permalink)
ColinRyan2 Registered User Join Date: Jul 2008 Posts: 21 OS: vista	Re: 1990 Chevy Truck Idle Problem what size motor is in it?

07-02-2008, 08:40 AM	#4 (permalink)
ColinRyan2 Registered User Join Date: Jul 2008 Posts: 21 OS: vista	Re: 1990 Chevy Truck Idle Problem Vaccum leak could be causing problem. It would also cause a code for Map sensor voltage to high.

07-02-2008, 08:43 AM	#5 (permalink)
JENSPORT Registered User Join Date: Aug 2007 Posts: 9 OS: win xp	Re: 1990 Chevy Truck Idle Problem Okay thanks for the input. Here are your answers the motor is a 5.7 350. The oxygen sensor has already been changed. I have looked around for Vaccum leaks but can't find any is there a spot that you can think I might want to focus on. This truck is driving me nuts. I call it Christine because things just seem to happen and we can't find an expanation.

07-02-2008, 08:51 AM	#6 (permalink)
dcraker Registered User Join Date: Jan 2008 Posts: 62 OS: Windows 2000	Re: 1990 Chevy Truck Idle Problem does it still do this on a cold start and before you actually have to drive anywhere? dual exhaust with dual catalytic converters? or a single catalylic converter? if you have one, then maybe you could try to disconnect it in front of the 'cat, then run it and see if it still does this, if it runs great (loud of course) then it could be your 'cat. OR, bang your 'cat converter some, to knock some of the debris loose, and run it and see if that changes the pitch any __________________ Thanks, David If you cover it in chocolate, it'll look better.

07-02-2008, 08:53 AM	#7 (permalink)
ColinRyan2 Registered User Join Date: Jul 2008 Posts: 21 OS: vista	Re: 1990 Chevy Truck Idle Problem You can focus around the intake manifold. around the throttle body and for any cracks in the vaccum lines running to the firewall or brake booster. Its sometimes hard to see or hear a vacumm leak. I my self will sometimes use a small spray of carb cleaner around any area were a vaccum leak may be. When you spray carb cleaner on a vaccum leak the engine ruff idle will sometimes clear up or cause the engine to run worse. it is normally noticeable if that carb cleaner is being sucked into the engine through a vaccum leak. BUT BE VERY CAREFULL WHERE YOUR SPRAYING. CARB CLEANER IS VERY FLAMMABLE!!!! DONT USE TO MUCH. JUST A LIGHT SPRAY. One other thing you can look at is the engine coolant temp sensor. Chevy small blocks and 4.3 liters are bad for running ruff when the sensor is reading wrong. Need to make sure what the engine temp sensor says is the actuall temp of the engine.

07-02-2008, 11:36 AM	#8 (permalink)
JENSPORT Registered User Join Date: Aug 2007 Posts: 9 OS: win xp	Re: 1990 Chevy Truck Idle Problem The truck will start right up and idle smooth for a couple minutes then you will hear it start to miss. The main problem is when it is gear and you are driving slow like out my driveway or in traffic. It will start to spit and sputter if you touch the gas peddle slightly. If you try like to hold the brake and give a little gas so it won't stall it works the opposite then it will definitely stall. If you let off the gas completely it will idle back up but still has a miss. The catalylic converter is hollowed out. I will try spraying the cleaner to find a leak. I already also tried the coolant sensor it has been changed.

07-03-2008, 07:07 AM	#9 (permalink)
ColinRyan2 Registered User Join Date: Jul 2008 Posts: 21 OS: vista	Re: 1990 Chevy Truck Idle Problem have you checked to see if the egr valve is stuck open. It will cause a rough idle at low rpms. would cause a stall when you first hit the accelerator. there should be no vaccum at idle going to the egr valve. if there is could be the egr valve solenoid. If there is no vaccum, while the engine is cool see if the plunger inside the egr valve is functioning. On that model you should be able to move the plunger inside the egr valve from the small opening on the outside. its should move up and down and you may hear a sucking sound because its a vaccum diaphram that lifts the plunger.

07-15-2008, 12:43 PM	#10 (permalink)
JENSPORT Registered User Join Date: Aug 2007 Posts: 9 OS: win xp	Re: 1990 Chevy Truck Idle Problem Well guys thanks for your input but I believe I finally figured it out. Knock on wood. The vacuum line from the egr valve was on the wrong thing on the map sensor. I feel like an idiot. All that aggrevation for 1 simple little thing. Thanks again for your help.

07-17-2008, 07:55 PM	#12 (permalink)
BUDFAN8 Registered User Join Date: Apr 2007 Posts: 193 OS: XP	Re: 1990 Chevy Truck Idle Problem hey i've searched for hours more than once because of a simple wire not being connected. i get in a rush ( mostly because people think stuff should be done yesterday) and it ends up taking longer. Bud.