Check Engine light not lighting up

[1987 GMC Jimmy]

That code maybe correlative to your low FP condition. It could be a vacuum leak like you said or an EGR issue, but a fuel deficit seems logical to me.

 

[dougbert]

I wouldn't trust a corroded distributor cap. When I was a teenager, we had a 64 mustang with a I6. Several guys diagnosed it internal timing damage since it mis-fired over and over again. So Dad decided to re-build the engine. It needed it

But once it was put back in the same mis-fires occurred. So the age old process was to question your assumptions.

Turns out the distributor cap had carbon tracks in it and was VERY hard to see them, but pulling the cap, turning off the lights and cranking the engine we could see cross firing even without the rotor being present. New cap and rotor and engine ran great. Doh!

So I learned to: Question my assumptions WHEN facing issues that make you go mmmmmmmmmmmm

dougbert

 

[HotRodPC]

Oh yes, without a doubt, you get carbon tracks or even carbon dust build up in the cap from a worn button that rides on the rotor cap, then the spark will be all over the place putting on a nice electrical show for them gremlins inside the cap to watch.

 

[adaml23]

Just to catch everyone up with this thread:

Got the gauges all working, and changed the fuel pump last weekend. Runs MUCH better now with the fuel pressure up to normal, but still has a mild stumble at low rpms under load that I'm chasing down. Blocked off the egr to see how it ran plugged up and it didn't change it. The stumble doesn't change weather the engine's warm or cold so I don't think it has anything to do with the CTS. Also changed out the cap and rotor yesterday with no changes. Don't know if I mentioned it or not, but the coil button was corroded as well, so I just ordered a new coil and will hopefully put it on soon to see if that fixes the stumble. Kind of running out of ideas tho if that doesn't fix it...

Got new/used tires on it today too. The old ones were dry rotted REALLY bad... 245/75/16 fit like a glove in the rear!

 

[chengny]

Check the TPS for tight clean connections, proper mounting and smooth operation. You have probably already looked for - and fixed - any air leakage under the throttle, but sometimes the evaporative emissions system can be overlooked. Check the hoses for splits and disconnects. Try pinching off the canister vent line to the carb.

 

[adaml23]

Just a follow up again... Got it fixed !!!

Turns out, the previous owner must have done something with the spark plug wires because cylinder #3 and 5 were switched!?! I replaced the wires a few weeks ago, but when I do them, I switch them one at a time so I don't have this happen... but I didn't even think to check if they were correctly hooked up. At least it was an easy fix, although it was driving me NUTS!!! Thanks for all the help guys.

 

[HotRodPC]

Yep, that'll certainly cause some stumbles. Like twice for every 2 engine rotations. So how does it feel to go from a V6 to a V8?

Thanks for updating the thread to resolution. Glad it was a cheap fix.

 

[adaml23]

Definitely much smoother now. Been driving around with pride now, instead of trying to feather the throttle sheepishly everywhere I go lol. This is my first big block so I don't have anything to compare it to besides my 79 4x4 400 sbc, and I know its no racecar, but wish it had a little more top end... and a 5th gear... Not thinking, I've now tried, multiple times, to shift into the imaginary 5th gear location...