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91 k5 Tbi. Runs different every time I drive it?

Sleeperatidle

Sleeperatidle

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As title says, I don’t know where to even start at, literally runs different every time I drive it, always idles good. Sometimes it will have a very little stumble to it under load, sometimes it misses, surges and just no power then all of a sudden full power under load.
Sometimes it will randomly die under load or just cruising. Turn the key off and on and starts right up.
Any place I should start to look? I’m confused. Been like this since I bought it years ago.
 
frankin5 said:
fuel filter, fuel pump, fuel pressure or ignition module.
Click to expand...
Thought of that too as a start, do you think ecu could be related since this isn’t a consistent problem? Just whatever day I drive it. Heck even driving it to work then driving it home it runs different.
 
ECU is most likely fine, unless internal engine mods were done. It will be one of the sensors or a fuel injection system issue most likely.

Something is sending bad signals or causing bad signals to go to ECU. The fact it still runs and corrects itself, means the ECU is working.

Check voltage at sensors, check injectors for dribbling, etc
 
Than
Bigb55 said:
ECU is most likely fine, unless internal engine mods were done. It will be one of the sensors or a fuel injection system issue most likely.

Something is sending bad signals or causing bad signals to go to ECU. The fact it still runs and corrects itself, means the ECU is working.

Check voltage at sensors, check injectors for dribbling, etc
Click to expand...
Thank you, I will start checking.
 
What's the basic state of tune? Recent plugs, wires, cap, rotor? Fuel filter?

One item that will lead to really inconsistent running is a worn throttle shaft on the tbi unit. Will end up with a vacuum leak that the ecm will end up chasing to get the fuel mixture right. Quick check besides feeling for slop by hand is shoot carb cleaner at the throttle linkage while it's running. If the engine speeds up, it's got a vacuum leak.
 
Bang your cat and listen for an internal rattle. Or if it is running poorly stop get out and check the exhaust flow. I the cat has broken inside a chunk could be intermittently be blocking the exhaust.
Another thing to check is the vacuum reading this will tell you the mechanical condition of the engine.
While the truck is running move wiring connectors and looms by hand note any change in the motor,
 
ZooMad75 said:
What's the basic state of tune? Recent plugs, wires, cap, rotor? Fuel filter?

One item that will lead to really inconsistent running is a worn throttle shaft on the tbi unit. Will end up with a vacuum leak that the ecm will end up chasing to get the fuel mixture right. Quick check besides feeling for slop by hand is shoot carb cleaner at the throttle linkage while it's running. If the engine speeds up, it's got a vacuum leak.
Click to expand...
I recently gave it a tune up cap, spark plugs and wires as well as fuel filter, I sprayed around vacuum lines to see if a change and nothing. I will try the throttle linkage. Never heard of that before will def try. Thank you!
 
Wes Harden said:
Bang your cat and listen for an internal rattle. Or if it is running poorly stop get out and check the exhaust flow. I the cat has broken inside a chunk could be intermittently be blocking the exhaust.
Another thing to check is the vacuum reading this will tell you the mechanical condition of the engine.
While the truck is running move wiring connectors and looms by hand note any change in the motor,
Click to expand...
No cat. No emissions where I live so straight exhaust to two borla mufflers. I haven’t stuck my vacuum gauge on it so that’s a idea there. You have any idea the vacuum range I’m looking at to be ideal?

Played with simple movement of the harness around sensors nothing changed.
 
Ground wires? Make sure the battery to the block, block to frame and block to tub are clean and tight. I added one from the frame to the tub, just for good measure.

What ignition control module is in it?
 
ideal vacuum would be 19-21 rock steady needle drop to 5-7 on quick snap and max above 20 when closed then settle 1-2 psi lower. Cam has a big effect on max vacuum at idle. those specs would be for a stock engine, in good tune, and mechanical condition.
low vacuum on stock engine, would be a sign of wear. Either stretch chain, worn rings. steady needle with a single drop regular would indicate bad valve. then compression test and leak down test to further pin down issues
 
Just please don't start throwing random parts at it. Some simple diagnostics can at least point you in the right direction.
- put a fuel pressure gauge on it and see if you get inconsistent or sporadic fuel pressure
- vacuum gauge, same as above...look at the difference between it running good vs. bad
- does it always run good when cold and the issue pop up randomly only after it is warm?
- do you have any O2 sensors on the exhaust?
- start checking voltages and resistance of the sensors....check them when it's running good and write the results down, then when it starts running bad do it again.
- the ECM can go bad...but it's probably one of the last things on the list to check......I've just seen a lot of people start replacing the highest dollar parts only to find out at the end it's shorted out wire or loose connector.......
I am not super-savvy on the TBI engines (assuming that is what we are talking about) so you will need to do some searching on trouble-shooting. YouTube can be a good source to walk you through the checks.......just keep in mind that just because it's on the internet doesn't mean it's true, so you need to look at different sources and use common sense.
My first suspicion on random issues like this is a bad wire or connection. I would visually check every wire related to the ignition and fuel and start tracing them back. Already mentioned but pull the grounds and clean them. Continue with the process of wiggling and moving wires and connectors around to try to get a change in how the vehicle is running....best bet is when the truck is running good mess with the wires and try to get it to run bad.
 
Once upon a time, I had a '91 that ran weird all the time - no power, auto trans shifted weird, idle'd good (didn't have a miss though), no check engine light, no codes. I eventually found an injector wire pinched under the air cleaner.
 
This injector wire issue has been the bane of many tbi weird problems. @shima found his were bad last year.
 
nvrenuf said:
Once upon a time, I had a '91 that ran weird all the time - no power, auto trans shifted weird, idle'd good (didn't have a miss though), no check engine light, no codes. I eventually found an injector wire pinched under the air cleaner.
Click to expand...
I do have an injector wire with some heat browning to it. But no pinches.
 
I would test the throttle position sensor . Several videos on YouTube if you don't have experience.
 
Sorry guys. I’ve been reading the help and not responded waiting on the truck. I took it to a shop rather than just throw many on “maybe’s”, among my problems I started draining the battery every 8hrs or so. I took it to a shop for last 4 weeks or so they been working on issues.
So first there was a small drawl on the battery due to all the light bar switches and always powered on CD player. Rewired to be on a master switch for lights and CD player was wired in correctly.

No draw when reading at the battery for a few minutes. Still dead battery after 8hrs.

mechanic found an old alarm system wired in under the dash. He took that out and found shorted out wires going to my headlight switch. Not surprising since head light switch turns on normal lights but also corner markers and chicken lights.
He used about an eighth of tank of gas driving it and never died driving. Maybe shorted wiring under the dash was the culprit.
He also fixed a few other little things.

I put at roughly 50 miles on it no issues besides engine stumbling. Check engine light came on and it starts running beautifully. I ran the code o2 sensor. So I unplugged it. Running much better, will replace it ASAP.

sorry everybody. I work ALOT and took it to a shop, I didn’t want to comment on anyone’s input due to not knowing if he can fix it or not.
 
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