Whats Wrong With My Truck?

Clowny · December 6, 2009

I have a 1991 S15 GMC Jimmy SLX. When I start it up, it will die right away. If I give it a little gas when it starts so that it won't die, it usually will have a very rough fluctuating idle after that. When i put it in gear and give it gas to go somewhere, it acts as if its being flooded. It just bogs down, starts popping and sputtering, and has little to no power at all. But if I let off the gas, it will idle normally for a few seconds and then back to the rough idle. I replaced the plugs, wires, cap and rotor recently with no change in how the truck acts. Any help and insight you can give me would be GREATLY appreciated. Thank you guys for taking your time ti help people out like this.

Nytemare · December 6, 2009

really hard to diagnose if your not throwing any codes.

That TB is very dirty...you should clean it....that would cause a rough idle, but not the bogging your describing.

Have you changed your fuel filter lately???

I'm not too sure but the plug at the back may be an IAC...but if it was gone it would throw a code.

Check for vacuum leaks too.

dlewis1111 · December 7, 2009

the CEL comes on sometimes but the truck acts like this all the time. I looked at the fuel filter when i first bought the truck and it looks brand new, plus the seller told me he changed it about 500 miles before i bought it. I'll see about changing it anyway. I just ran out to autozone and picked up a code reader, fuel filter, and fuel injector cleaner. No codes popped up on the reader when I tested it. I cant change the fuel filter yet cause it got dark and has started snowing. Hopefully itll clear up enough tomorrow.

Go to Advance Auto parts and get them to read the codes with their machine - for free -. Your stored code is not retreivable with the one you bought. Changed the o2 sensor yet?

dlewis1111 · December 7, 2009

I have a 1991 S15 GMC Jimmy SLX. When I start it up, it will die right away. If I give it a little gas when it starts so that it won't die, it usually will have a very rough fluctuating idle after that. When i put it in gear and give it gas to go somewhere, it acts as if its being flooded. It just bogs down, starts popping and sputtering, and has little to no power at all. But if I let off the gas, it will idle normally for a few seconds and then back to the rough idle. I replaced the plugs, wires, cap and rotor recently with no change in how the truck acts. Any help and insight you can give me would be GREATLY appreciated. Thank you guys for taking your time ti help people out like this.

Stop what you are doing right now and go to the parts store to purchase the haynes manual that has the exploded diagrams of your truck. Yes, it has an IAC on the right side forward valve cover (the black box with the vaccuul tube), and the fact that you didn't know this warrants the need for the book so that you can bone up and fix this sucker.

That isn't the IAC valve.

On the Throttle body, right side rear, just behind the TPS.

That the map sensor isnt it? It has been a day or two for me on these.......

Clowny · December 8, 2009

Update: So apparently in my haste and fervor to figure out the probelm and fix it, I forgot how to count. It is throwing code 13, not 12. So the o2 sensor is reading lean. I picked up a o2 sensor today and went to swap it out, the old one is too old and stuck in there for me to remove with the tools I have. So hopefully soon, I can get the truck to a mechanic buddy to have him swap it for me. Correct me if I am wrong (which I probably am) but that explains the bogging, popping, and sputtering, but not the idle issue since the vehicle doesnt use the o2 sensor til it reaches operating temperature.

minibike132 · December 7, 2009

I have a 1991 S15 GMC Jimmy SLX. When I start it up, it will die right away. If I give it a little gas when it starts so that it won't die, it usually will have a very rough fluctuating idle after that. When i put it in gear and give it gas to go somewhere, it acts as if its being flooded. It just bogs down, starts popping and sputtering, and has little to no power at all. But if I let off the gas, it will idle normally for a few seconds and then back to the rough idle. I replaced the plugs, wires, cap and rotor recently with no change in how the truck acts. Any help and insight you can give me would be GREATLY appreciated. Thank you guys for taking your time ti help people out like this.

Stop what you are doing right now and go to the parts store to purchase the haynes manual that has the exploded diagrams of your truck. Yes, it has an IAC on the right side forward valve cover (the black box with the vaccuul tube), and the fact that you didn't know this warrants the need for the book so that you can bone up and fix this sucker.

That isn't the IAC valve.

On the Throttle body, right side rear, just behind the TPS.

That the map sensor isnt it? It has been a day or two for me on these.......

The one with 2 vacuum lines? I'm pretty sure that is the valve that controlls the egr vacuum, the map is bolted to that valve just inboard of that valve.

dlewis1111 · December 7, 2009

I have a 1991 S15 GMC Jimmy SLX. When I start it up, it will die right away. If I give it a little gas when it starts so that it won't die, it usually will have a very rough fluctuating idle after that. When i put it in gear and give it gas to go somewhere, it acts as if its being flooded. It just bogs down, starts popping and sputtering, and has little to no power at all. But if I let off the gas, it will idle normally for a few seconds and then back to the rough idle. I replaced the plugs, wires, cap and rotor recently with no change in how the truck acts. Any help and insight you can give me would be GREATLY appreciated. Thank you guys for taking your time ti help people out like this.

Stop what you are doing right now and go to the parts store to purchase the haynes manual that has the exploded diagrams of your truck. Yes, it has an IAC and the fact that you didn't know this warrants the need for the book so that you can bone up and fix this sucker. While you are there, get a new PCV valve and both of the fuel filters (if it has two). How many miles are one it? Are you setting the timing according to the manual? There is an internal spring on the top of the throttle body that deals with idle characteristics. It is non adjustable and when it goes out you either have to replace the top of the throttle body assembly or the entire thing. Your two injectors are replaceable, and they will come in a kit to overhaul the throttle body. Are you pinning out the diagnostic connector to make it flash the codes at you? Also, parts stores will do a free scan to check for codes in the hopes of selling you the replacement parts. Use this, its free.

Nytemare · December 7, 2009

I have a 1991 S15 GMC Jimmy SLX. When I start it up, it will die right away. If I give it a little gas when it starts so that it won't die, it usually will have a very rough fluctuating idle after that. When i put it in gear and give it gas to go somewhere, it acts as if its being flooded. It just bogs down, starts popping and sputtering, and has little to no power at all. But if I let off the gas, it will idle normally for a few seconds and then back to the rough idle. I replaced the plugs, wires, cap and rotor recently with no change in how the truck acts. Any help and insight you can give me would be GREATLY appreciated. Thank you guys for taking your time ti help people out like this.

Stop what you are doing right now and go to the parts store to purchase the haynes manual that has the exploded diagrams of your truck. Yes, it has an IAC on the right side forward valve cover (the black box with the vaccuul tube), and the fact that you didn't know this warrants the need for the book so that you can bone up and fix this sucker.

That isn't the IAC valve.

On the Throttle body, right side rear, just behind the TPS.

That's what I thought was the IAC. IAC is not usually vacuum controlled.

Either way...it would throw a code 35 if it was gone....so unless there is a code I doubt its the problem.

My bet is on fuel....weather it be pump, regulator, filter..or whatever.

Second bet would be on a clogged converter.

After that its down to chasing vacuum leaks...removing the EGR and replacing it either with a new one or with a blockoff plate...

If you have a code however...than its a much easier troubleshooting procedure.

minibike132 · December 8, 2009

Update: So apparently in my haste and fervor to figure out the probelm and fix it, I forgot how to count. It is throwing code 13, not 12. So the o2 sensor is reading lean. I picked up a o2 sensor today and went to swap it out, the old one is too old and stuck in there for me to remove with the tools I have. So hopefully soon, I can get the truck to a mechanic buddy to have him swap it for me. Correct me if I am wrong (which I probably am) but that explains the bogging, popping, and sputtering, but not the idle issue since the vehicle doesnt use the o2 sensor til it reaches operating temperature.

Low fuel volume explains it all, just because the o2 sets a code doesn't mean there is a problem with it. It's telling you the engine is lean, maybe the sensor is right and you are lean.

Clowny · December 8, 2009

i will have the mechanic check the fuel pressure while he has the truck to see about that.

Matt_ · December 8, 2009

i will have the mechanic check the fuel pressure while he has the truck to see about that.

See post #2.

Clowny · December 7, 2009

the CEL comes on sometimes but the truck acts like this all the time. I looked at the fuel filter when i first bought the truck and it looks brand new, plus the seller told me he changed it about 500 miles before i bought it. I'll see about changing it anyway. I just ran out to autozone and picked up a code reader, fuel filter, and fuel injector cleaner. No codes popped up on the reader when I tested it. I cant change the fuel filter yet cause it got dark and has started snowing. Hopefully itll clear up enough tomorrow.

jro909 · December 8, 2009

the 91s take a paper clip and you jam it into the computer port and turn the ignition to run if i remeber right to get the codes, then you crack out the giant phone book of codes or google it lol..just from my experience from my 91 s-10

i second the exaust restriction, i drove with no o2 sensors with no problems but then again my exaust was a piece of pipe spot welded on that ended just behind the cab lol

Nytemare · December 8, 2009

i will have the mechanic check the fuel pressure while he has the truck to see about that.

See post #2.

LOL...they never listen Matt...I'm sure we said fuel about 5 or more times so far...LOL

Remember AFS...the basics to trouble shooting. Air Fuel Spark

After that you start going with sensors...

Lean mixture is a sign of bad fuel pressure/flow. So plugged filter, pump on its way out, leaking line...bad Pressure regulator. Have them all checked.

Nytemare · December 7, 2009

the CEL comes on sometimes but the truck acts like this all the time. I looked at the fuel filter when i first bought the truck and it looks brand new, plus the seller told me he changed it about 500 miles before i bought it. I'll see about changing it anyway. I just ran out to autozone and picked up a code reader, fuel filter, and fuel injector cleaner. No codes popped up on the reader when I tested it. I cant change the fuel filter yet cause it got dark and has started snowing. Hopefully itll clear up enough tomorrow.

Odd...it should have stored a code....you may have to bring it somewhere to get the codes read that are in memory..I don't know if those cheapies do that....I've never used one...always use the tech II or MAC/Snapon's version.

If the IAC is gone it will definitely throw a code 35.

Just thought about this.......

Did the guy you bought it from live on a farm or anything....and was he driving it when you bought it.

You may have a nest in your air intake. does it drive and idle rough with the intake piping removed???

A Clogged intake usually exhibits these exact symptoms.

Or not...you don't have piping on a 91 do ya....LOL

Clowny · December 7, 2009

It acts the same with or without the intake tubing in place. Ive actually been driving this truck for the last 2 months or so. It was running fine until a week or 2 ago. Thats when it really started having these problems.