Rpm's go up and down off idle

[87ChevyR10]

And I don't mean just pressing on the pedal to slightly raise Rpm's. I held the motor at 2000 Rpm's and it fluctuates about 50 Rpm's.

I am replacing both temp switch and map sensor tomorrow. Also looking at rebuilding the tbi, too.

Anything else I should consider?

 

[1987 GMC Jimmy]

Maybe ignition stuff. Check the cap and rotor to see if they still look good, or if they look worn out. Are your wires fairly new? If all this stuff is good, that's great. I don't know if you want to get to playing with the ignition coil and the innards of the distributor. How's your vacuum situation? I'm not a smoker and don't wanna be, but I did get a cheap cigar, and I blew through the whole emissions catalyst and found a few vacuum leaks. I'm just thinking cheap/easy stuff to check off your list. Good vacuum, spark, and air flow, plus you know what you want to do with the fuel and sensor aspects... That should pretty much cover it. Oh, it may be a good idea to invest in one of those Schrader valves that screw into the TBI so you can check your fuel pressure. Summit sells them, as do others, I'm sure.

 

[87ChevyR10]

Replaced temp switch and MAP sensor. Seems better, but not 100% convinced. I pretty sure the spark plugs and wires need replacing; plus, I need to get proper spacers for the wires.

The symptoms I'm getting is it'll rev up a bit and then fall, rev a bit then fall over and over again. It's doing this at most RPM ranges, just not at idle.

I'm rebuilding the TBI today. Hopefully I won't jack that up. When I pull it off, do I need to worry about fuel spraying out anywhere?

Thanks!

 

[1987 GMC Jimmy]

I don't think it'll be bad. If you ran it recently, there will be some leakage out of the main line, but it shouldn't spray. Those lines can be a pain sometimes, but that's anything with steel lines. The rebuild shouldn't be bad. How many miles are on the truck?

 

[Georgeb]

You will also want to clean up all your grounds on the engine. One by the water neck and one on the back if the passenger cylinder head.

 

[87ChevyR10]

I don't think it'll be bad. If you ran it recently, there will be some leakage out of the main line, but it shouldn't spray. Those lines can be a pain sometimes, but that's anything with steel lines. The rebuild shouldn't be bad. How many miles are on the truck?

It's got 92,000 to 93,000 miles on it.

 

[87ChevyR10]

Well, this is why I hate doing work myself, I break stuff. I had everything disconnected from the TBI except for the fuel line on the rear left (facing engine). I worked at it until I thought it was free. Well, it was: free as in two pieces. I successfully twisted the line into two pieces. The right side was EASY. Didn't take any effort at all.

So, now that the TBI is off, I still cannot get the 5/8" section to unscrew from the 3/4" piece. Once I am able to separate the 2, what are my options for fixing the fuel line? Could I cut the twisted bit off and run a rubber fuel line to the TBI with a 5/8" NPT fitting? Or am I going to have to have a shop make a tube for me?

Thanks!

 

[87ChevyR10]

THANK GOD for Doorman! I found an 18" length of tubing which comes with a brass coupler. All I gotta do is cut off the twisted bit under the hood and cut the new piece down to size and couple them together!

 

[1987 GMC Jimmy]

Same ALMOST happened to me. I saw what I was doing before I twisted it off and twisted it back. There's still a dent, but it works okay. Sorry to hear that but glad you found a fix.

 

[87ChevyR10]

Got the TBI put back together. They give enough extra gaskets to make your own spacer for both the base and the injectors. For whatever reason, the base gaskets come with 3 different pairs: 1st set covers IAC and between the butterflies; 2nd set covers the IAC, but does not cover between the butterflies; 3rd set does not cover IAC, but covers between the butterflies. Then there is a black spacer for something. Looks to go between manifold and base of TBI, but it doesn't cover up the IAC.

I'm done for the night. Tomorrow, I get to fix the fuel line and put it all back together.

 

[87ChevyR10]

Yeah, it's all back together! The fuel line fix I did was the worst part. Due to the specific bends the lines had, it took some time to get it all lined up.

Getting ready to go for a test drive.

 

[87ChevyR10]

Just got back from the test drive and throttle response seems snappier. The Rpm's seem more stable, too. I also did the lid flip.

I'll have to post a pic of the accelerator diaphragm when I get the chance. It wasn't torn, but rusty as heck as is the spring.

Glad that's done.