1996 Chevrolet Truck • 224,763 miles

96 chevy c2500 5.7 vortec.

just installed the motor. Replaced fuel filter, fuel pump, truck starts and idls fine. But when put in gear. Rpms drop to about 500-600, lights dim and then picks back up.

what could be the issue?
November 14, 2012.

The Idle Air Control Valve, IACV could be bad. It keeps the idle steady in cold starts and changes in engine load. So, the colder it gets the worse the problem may get.

Ok that is what I might be thinking as well. Just curious. I just cleaned off the iacv last night as it was covered with carbon build up.

Should I take off the tb and clean it as well? Is it a possibility that my alternator is going out?

Is there a possibility that this could also be caused by a bad cap and rotor, plugs, wires, or bad distributor?

I greatly appreciate the advice. Im a DIY guy and prefer to do what I can before taking it to someone.

Nov 14, 2012.
When you clean the throttle body make sure you clean the ports from the outside going in towards the center. That way carbon buildup will not get further pushed into ports. You should also check that the IACV solenoid works by leaving it unbolted but hooked to the electrical connector. Have someone start it and see if it moves the plunger.
It could be any of the issues you mention but the iacv is the best place to start.

Maybe this walk through diagnostic might help you out some. Just use the link to get to it;

Wow, you are very timely with your responses. I greatly appreciate that. I have fiddled with this truck for over a week now trying to get everything back up to par. Lucky for me I have an extra alternator sitting around so I can change that at the same time I clean the throttle body. I also have another IACV sitting around so before installing it I will do my best to make sure the tb and current IACV ar cleaned up and looking goo. I will also double check to see if I have a vac leak somewhere. It does seem to have a whistle from somewhere, but then again I haven't driven this truck in almost a year and really don't remmeber what it sounds like. I will try a few things tomorrow and report back with what I have found and if problem becomes resolve with trying a few things out.

Thank you again for timely response. Knowledge is great. Hope to share with someone else if they run into the same issues.

Nov 14, 2012.
If the truck has sat for a long time and it did not have much gas in it, add as much gas as you can because gas turns to a varnish very quickly and can clog filters and injectors. You might want to run some BG44k once you get it running. You have to go to a car dealership parts counter to get it and it costs $20 a can. However 2 cans will clean up the filthiest of motors. It can tend to make the engine run a little funny while it is cleaning out stuff, but by the time you run a third tank, each can treats a tank of gas, everything clears up and you will notice a difference.

Thank you!

I have found that my rpm drop was caused by being out of time a little. Still out of time as my check engine light is still throwing a code, however does run better at idle and got it stable enough not to drop rpm when in gear. Took it for a test drive and ran like a top.

I still think I will run some of the BG44K through it and see if it does run a little better and get some of the gunk carbon build up out. Motor ran fine before it was pulled and am happy to see that it is still a power house. Now if I can get it right in time where it needs to be, it will be ready to sale and someone else can enjoy it as I have a 97 1 ton dually as my DD with a 7.4. Thank you for advice.

I have also cleaned the tb and checked for vac leaks. Everything seems to be golden from there.

Nov 14, 2012.
Wow, you did a lot of work real fast. I take a long time, too long, to get things done.
I think you are so close that the fresh gas and BG44K will get it in excellent working order. Remember it may run funky while the BG cleans out deposits and sludge and bad gas, varnish, before it settles down and runs like it going to.
Glad it turned otu okay.

Dr. C