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1991 K5 TBI Issue

Rhinodrvr

Rhinodrvr

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Hello friends.

Challenge for me today. My 91 K5 developed a rough idle about 3 weeks ago. I can tell it is loading up to the smell of fuel. I replaced the fuel injectors about 2 years ago and it has been running great.

So today we did a tune up...plugs, distributor car and button and replaced the ignition control module. Now it seems worse. I am starting to wonder if the TBI unit is bad and needs to be replaced. Anyone replaced their stock TBI unit for any type of upgrade?

My K5 is mostly stock and I am trying to keep it close but I am not opposed to some type of update/upgrade.

The 91 seems to be a bastard of technology and parts. I find some parts are obviously form the square body era but some of the drive train components seem to have been brought over from the 88-2000 era trucks.

I am getting frustrated and would appreciate any ideas, thoughts or advice.

Thanks!!!
 
No. The Service Engine Soon light has only come on once and i think that was more related to the engine lope/miss. It is a stock motor with no mods but it acts like it has an oversized cam in it. The RPMs dropped so low at one point all of my lights came on...BRAKE, SERVICE ENGINE SOON, ETC.

I don't even have a OBD I scanner. Probably need to get one of those..:)
 
No, you don't need a scanner, you can use a paperclip between terminals A&B on the ALDL connector.

Did you pay close attention to all your vacuum lines and fittings? Any vacuum leak is going to cause issues. You can look at the injector spray pattern as it runs as well, looking for a nice cone, no drips.
 
dyeager535 said:
You can look at the injector spray pattern as it runs as well, looking for a nice cone, no drips.
Click to expand...

Check that, also when is the last time you cleaned/rebuilt the TBI? IAC functioning normally? Anything change 3 weeks ago? Is the idle rough cold? Warm? Both? Surging?
 
Hello I purchased my 89K5 it ran and drove pretty decent. I live in Jersey and drove it home from Philadelphia. I know the engine had a bad head gasket. I brought heads to machine shop and started to clean and rebuild TBI. I was surprised how gummed up it was inside. Passages were partially clogged but yet it ran.

It was fairly easy to rebuild. Cleaned passages with fiber brushes. Rebuild kit from Advanced Auto. The one item I didn’t repair was the distributor. It had surface rust and pickup coil wires were cracked. I cleaned it best I could coated cracked wires with silicone and replaced it back. It ran great for quite awhile until past fall. It started with erratic idle.

The IAC was good the pigtail connector was bad. Replaced it idle was excellent. Then months after a backfire. WOW that hurt. It was in part the distributor. I rebuilt it. I also replaced all the connections to distributor. I then found the oil pressure switch was leaking and forcing oil into it’s connector and saturated the wires. I cut back the wires and replaced connector and new switch.

The distributor was rebuilt with totally new parts. Shaft with wheel, pickup coil, reluctor, cap, module all GM except reluctor. Topped it off with new coil. It runs great timing is dead on. I had a 99 Ford 7.3 diesel and the oil pressure switch went bad and saturated connector and wire with oil. You can’t tell by looking at it. It caused erratic idle.

Once changed it ran great. So somewhere down the line it’s possible your truck has one of same issues. My truck is running nice. I just redone all the ground wires going from engine to body firewall. This was long but it might help. I had a lot of help from K5 Members.
 
Play in the shaft of the TBI butterflies can cause an erratic idle. Open the throttle a little, then "wiggle" the shaft and see how much play is there.
 
The only thing that changed was cleaning the motor off . Light spray and simple green. Done this to this motor before and never had this reaction.

After the tune up it got worse. When the motor is cold (first start) is seems to run smooth for about 5 minutes. After that it moves to a surging at low MPH/RPMs. At a stop it really lopes and the RPM drop. I'm probably going to pull the TBI, rebuild it and replace the injectors again. They were replaced about 2 years ago but will do what I have to do.
 
I had a shot MAP sensor cause a similar issue with my rig, but I knew when I did. I was stabbing in a new dizzy and was off. I ended up with a pretty good backfire come out the intake and it was more than my 28 year old map could handle.
 
Maybe loose bolts holding down TBI. Taking in air between TBI and manifold.
 
remove air cleaner look down throttle bore while opening throttle, engine off, look for gasket material protruding into bore. TBI can sometimes suck the base gasket. this will cause a vacuum leak.
 
Rhinodrvr said:
The only thing that changed was cleaning the motor off . Light spray and simple green. Done this to this motor before and never had this reaction.

After the tune up it got worse. When the motor is cold (first start) is seems to run smooth for about 5 minutes. After that it moves to a surging at low MPH/RPMs. At a stop it really lopes and the RPM drop. I'm probably going to pull the TBI, rebuild it and replace the injectors again. They were replaced about 2 years ago but will do what I have to do.
Click to expand...
Same symptoms, I had rebuilt my tbi twice convinced somewhere around there my problem was, but in the end it indeed was my MAP sensor.
 
I think I changed about everything on the engine by now. It’s running good. I’m going to change out wiper high beam switch that’s in the column.
 
Rhinodrvr said:
So today we did a tune up...plugs, distributor car and button and replaced the ignition control module. Now it seems worse.
Click to expand...
Try the old ignition module as sometimes new ones are bad out of the box. Also check all the spark plugs to see if you cracked the porcelain on one. It's also possible something was just put together wrong in the distributor.

Of course anytime you mess around old vacuum lines there is chance of breaking one or knocking one off. I like to listen to all the lines and TB with a stethoscope, but some people prefer shooting carb cleaner or propane all around. A speed density engine will always run bad with a vacuum leak.
 
Rhinodrvr said:
Hello friends.

Challenge for me today. My 91 K5 developed a rough idle about 3 weeks ago. I can tell it is loading up to the smell of fuel. I replaced the fuel injectors about 2 years ago and it has been running great.

So today we did a tune up...plugs, distributor car and button and replaced the ignition control module. Now it seems worse. I am starting to wonder if the TBI unit is bad and needs to be replaced. Anyone replaced their stock TBI unit for any type of upgrade?

My K5 is mostly stock and I am trying to keep it close but I am not opposed to some type of update/upgrade.

The 91 seems to be a bastard of technology and parts. I find some parts are obviously form the square body era but some of the drive train components seem to have been brought over from the 88-2000 era trucks.

I am getting frustrated and would appreciate any ideas, thoughts or advice.
Click to expand...
 
Hello I rebuilt mine using kit from Advanced Auto. It was plugged up but ran. Other then cleaning it up it was pretty basic project. I think it’s more simpler to work with than a none injection model carb. Just be careful of fuel line threads.
 
nbrown2500 said:
Have any of you guys done business with these guys? I was going to order a TBI unit from them.

https://throttlebodyinjection.com/
Click to expand...

Seems like a lot of darn money for a $20 part.

That said, I haven't added the cost for components up, nor the cost of bushing the throttle shaft, which can be an issue with high mileage units. GM used that TBI unit from 1987-1995 on trucks, so they are by no means rare, at least talking small block. The big block units are somewhat more rare, but doesn't matter unless you NEED the increased airflow. Not a good idea if not needed.
 
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