Small block chevy stumbles when it gets hot?

Question

I have a 1989 350 in a Silverado that stumbles when it gets hot. It s a tbi truck so I don t know if that would make any difference. In the past year or so I ve replaced the distributor, coolant temp sensor, and the cap and rotor. I tried the distributor module and the coil but neither of them fixed it so I returned them. I m thinking it s some sort of ignition problem because it only does it when it gets hot but I m not an automotive expert by any means, it s just a hobby. It runs fine up until it gets above the 200 mark which is where the truck is supposed to run. I don t believe it s out of time because the distributor is tight but I don t have a timing light to be certain. Any suggestions or anyone with a similar problem? Thanks so much!

Answer 1

Heat or coolant temp related problems can be due to numerous issues such as bad ignition coils or module, bad cam/crank sensors, etc.. which may work fine when cold but fail when heated up in the engine bay due to high internal resistance. And those can cause the truck to lack power, stumble or stall, etc... but hard to pin down on older OBD-I systems. Other systems like the EGR or EVAP system also operate only when the engine is warmed up and may cause issues if not working properly.



I'd check the EGR and EVAP systems first given the age of the truck but it could indeed be other ignition issues. Make sure the EGR valve is operating/opening as it should when engine is warmed up and check the solenoid that controls it...also a fuse in the circuit. Clean out the EGR / manifold passages if needed...some techs use an old section of speedo cable or drive cable for a weed trimmer , etc..chucked into a drill for cleaning passages along with a solvent to loosen carbon crud ...a clogged passage may keep the EGR system from working as it should to lower combustion chamber temps which can in turn cause a rough engine/stumble.



Also check the EVAP system for leaks... a leak in the system can cause poor power, stumble, etc.. once the EVAP purge valve/solenoid opens as the engine is warmed up to operating temps. Locate the vacuum hose from the engine manifold or TBI unit to the EVAP purge valve and clamp it off or remove the hose/cap the vacuum nipple...road test to see if the truck runs better warmed up. If so, then check the EVAP lines/system for leaks.



Bad ignition related parts can also be an issue. If possible, check for weak spark with an inline/ visual spark tester on a few plugs once the engine warms up...compare to spark with a cold engine. If the spark looks weak, then perhaps test or replace the ignition module/coil and possibly the cam or crank sensor need to be tested for high resistance once warmed up. Not always easy to replicate a cold or hot engine condition though for testing but if possible, test under various engine temps.



Spark tester...

https://shop.advanceautoparts.com/p/dorman-conduct-tite-electrical-tester-in-line-spark-plug-checker-86579/22144233-p?c3ch=PLA&c3nid=22144233-P&c3apidt=23534110302&gclid=EAIaIQobChMI7cPx0pue1gIViSaGCh3jPwdrEAQYAiABEgIrqvD_BwE&gclsrc=aw.ds



http://easyautodiagnostics.com/gm/4.3L-5.0L-5.7L/distributor-mounted-icm-tests-1



http://troubleshootmyvehicle.com/gm/4.3L-5.0L-5.7L/how-to-test-the-crank-sensor-2



EGR and EVAP systems...

http://troubleshootmyvehicle.com/gm/4.3L-5.0L-5.7L/stuck-open-egr-valve-tests-1

http://arrc.ebscohost.com/searches?car_uuid=1031140&fa_uuid=64bf23dd-8d00-1ce5-b989-a8eb286ac873_25&filters%5Bservice_information_type_path_facet%5D=root%2Fmrt%2Fsit%2Fp5_repair&keywords=egr



http://arrc.ebscohost.com/searches?car_uuid=1031140&fa_uuid=64bf23dd-8d00-1ce5-b989-a8eb286ac873_20&filters%5Bservice_information_type_path_facet%5D=root%2Fmrt%2Fsit%2Fp5_repair&keywords=evap

Answer 2

We washed the engine of our car once at a do it yourself car wash place. Afterwards we noticed that we would have a bit of misfiring or juggling of the engine once it got hot. After much headaches and searching, we found that the cold water from the car wash had cracked the porcelain of one of the spark plugs. Only when it got hot would the spark travel on the outside of the plug and short onto the engine block. We only found it after a point of desperation and replacing all of the spark plugs! We didn't check it first because just a week before we had already replaced the spark plugs as a routine maintenance. So we thought that our problem was elsewhere since we had new spark plugs. Little did we know that we had washed the engine while the plugs were already really hot from driving the car. New or not, one of them cracked from the cold water hitting it!



Don't check your spark plugs, just replace them! That crack was not seen until after we had replaced the plugs and the problem fixed itself. We then looked closer at them. It was a very thin hairline crack!

Answer 3

It's old, many things could be wrong. Some thoughts:



100% you need to be able to confirm timing is set correctly.



EGR, take a soda can and cut a little temporary block off plate to bolt under it to see if that changes it. Just unhooking the vacuum isn't always enough because of how that valve works.



Grounds near thermostat need to be good, clean.



Alternator leaking A/C voltage is fairly common on those. Warm it up and the unplug the small wire at alt and take a short drive to tell.



If a fuel injector is failing TBI get weird and worse as it warms up and the computer takes control of the fuel mixture. Notably less power is normally observed warm. I test that with a scan tool looking at fuel trim or with a timing light to catch the injector spraying while running. I have no idea how you could test that without proper tools.