This is a long shot but here we go. I have an 88 k2500 with the 350tbi and 700r4. i have made a few posts asking for help with my pos obs and this post has the same theme. I have been battling low power off and on since august of 2023 and I thought that I finally fixed my power issues when i replaced the egr with the correct one, the Egr solenoid, and the knock sensor which all three helped a lot but it’s still not running right. I know it’s not running right because of 2 things, 1) it does some weird surging thing when cold and below 2500 rpm, after 2500rpm it takes off. 2) I happened to drive my dad’s stock 1990 k1500 with same motor, transmission, and rear gear up a hill that I drove my truck up earlier that same day and i realized that in dad’s truck, the converter didn’t unlock (mine unlocked right at the beginning of the hill)and I was not on the gas petal nearly as much. And overall driving it seems like dad’s truck isn’t struggling like mine is. And to answer one comment, I am aware that a 2500 weighs more than a 1500, I just don’t believe that this difference in weight can make this much of a difference in performance. And I don’t have access to a scanner. I have done a whole bunch of work to this thing that I can list if needed. Sorry for the essay, thanks in advance for the help.
88 5.7 low on power
- Thread starter thered88
- Start date
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Komet
I'm Awesome
Find access to a scanner, that would be an easy way to see what's really going on.
I mean catalytic converter
Well, I always did a quick check by having a second person in the driver's seat and held my covered hand at the tailpipe and see if it had good even full flow when the engine was throttled up. Or I checked the engine vacuum at idle and rev up and hold the throttle at a higher rpm. Good vacuum at idle, see it drop when rev but then climb and maintain good vacuum when hold the higher throttle at cruise. Or do a back pressure check:
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someotherguy
Truly Awesome
Fuel pressure as well as base timing is what I'd check first, and then I'd check that the timing is advancing when the bypass connector is plugged back in.
Many people that aren't used to working with these ignitions may adjust the timing "by ear" or have the distributor out to do intake gaskets, and not know how to correctly set the base timing.
There's a tan wire w/black stripe in the engine bay that has a disconnect for this purpose. On your '88 you'll likely find it behind the plastic cover on the passenger side of the firewall. Typical location on most 88-93 GMT400's.
Engine warmed up, disconnect that wire, then throw a timing light on it and set the distributor for 0° BTDC. Snug it down, re-check that it didn't move, still at 0° - then plug that connector back in, and it should jump up roughly 12° or so and expect it to not hold steady; it will move around a little bit as the ECM is constantly adjusting timing advance.
It helps to find the mark on the balancer and paint it white with a paint marker, and wipe any grease off the timing tab on your timing cover first. As you can see the wider triangle mark has 0° at the center of it.
If you don't see the timing advance when the wire is plugged back in, then suspect the ignition module in the distributor.
Next up? Not sure what all else you've done but if your CTS (coolant temperature sensor) on the intake manifold is old, or the connections to it are ratty, especially the grounds at/near the thermostat housing, it could be incorrectly reporting the coolant temperature to the ECM. When these sensors go bad or the circuit has too much resistance due to corrosion at the connections, they read colder than actual temp, which causes the ECM to richen the fuel mixture too much and this will result in poor drivability, too. A scan tool will show you what it's telling the ECM and you can compare that to temps read at/near the thermostat housing with an IR temp gun, because you can't really trust the dash gauge. You can check the sensor itself with an ohmmeter. If it looks really old, I say go ahead and replace it because they all eventually go bad with age.
The surging could be a possible vacuum leak. TBI base gasket, vacuum fitting or hose loose/broken, etc. An unlit small propane torch slowly moved around the engine while running will increase the idle speed when you find the leak and it sucks in the propane. You can also do this with a spray can of carb cleaner but I find the propane easier to control than any wet chemical that you spray.
These things cover a lot of the bases of your complaint. I even had a customer years ago bring me his '95 Tahoe because he was sure his engine was junk and was looking to buy a replacement. Base timing was WAY off, CTS was very bad, EGR was bad, and sensor grounds were green corroded. After the timing was set, CTS replaced, and sensor grounds cleaned up - it ran nearly like new. I gave him info on ordering an EGR and that took care of his code 32.
Richard
Many people that aren't used to working with these ignitions may adjust the timing "by ear" or have the distributor out to do intake gaskets, and not know how to correctly set the base timing.
There's a tan wire w/black stripe in the engine bay that has a disconnect for this purpose. On your '88 you'll likely find it behind the plastic cover on the passenger side of the firewall. Typical location on most 88-93 GMT400's.
You must be registered for see images attach
Engine warmed up, disconnect that wire, then throw a timing light on it and set the distributor for 0° BTDC. Snug it down, re-check that it didn't move, still at 0° - then plug that connector back in, and it should jump up roughly 12° or so and expect it to not hold steady; it will move around a little bit as the ECM is constantly adjusting timing advance.
It helps to find the mark on the balancer and paint it white with a paint marker, and wipe any grease off the timing tab on your timing cover first. As you can see the wider triangle mark has 0° at the center of it.
You must be registered for see images attach
If you don't see the timing advance when the wire is plugged back in, then suspect the ignition module in the distributor.
Next up? Not sure what all else you've done but if your CTS (coolant temperature sensor) on the intake manifold is old, or the connections to it are ratty, especially the grounds at/near the thermostat housing, it could be incorrectly reporting the coolant temperature to the ECM. When these sensors go bad or the circuit has too much resistance due to corrosion at the connections, they read colder than actual temp, which causes the ECM to richen the fuel mixture too much and this will result in poor drivability, too. A scan tool will show you what it's telling the ECM and you can compare that to temps read at/near the thermostat housing with an IR temp gun, because you can't really trust the dash gauge. You can check the sensor itself with an ohmmeter. If it looks really old, I say go ahead and replace it because they all eventually go bad with age.
The surging could be a possible vacuum leak. TBI base gasket, vacuum fitting or hose loose/broken, etc. An unlit small propane torch slowly moved around the engine while running will increase the idle speed when you find the leak and it sucks in the propane. You can also do this with a spray can of carb cleaner but I find the propane easier to control than any wet chemical that you spray.
These things cover a lot of the bases of your complaint. I even had a customer years ago bring me his '95 Tahoe because he was sure his engine was junk and was looking to buy a replacement. Base timing was WAY off, CTS was very bad, EGR was bad, and sensor grounds were green corroded. After the timing was set, CTS replaced, and sensor grounds cleaned up - it ran nearly like new. I gave him info on ordering an EGR and that took care of his code 32.
Richard
Last edited:
Great info.
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