Hello,
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I am having a strange issue with my 1994 Chevy Silverado. It has 187,000 miles, with a stock 5.7L 350. For some reason when its idling it “lopes”. It reminds me of a hot rod engine with a big cam. LoL. Anyways, it runs great when your driving it. It has a ton of power and will roast the tires if you want it too. In the past 3 months I have rebuilt the engine because it was burning oil and leaking oil all around the intake, it also leaked antifreeze around the intake. So the block and heads were rebuilt by a local machine shope, with new valve guides, seals, piston rings, bearings, timing set, new cam, etc.. I got the engine back and dropped it back in the truck. After installing all the electrical components, I fired her up and she ran like woman wearing lead boots. So I began troubleshooting and ended up replacing the following parts. Throttle Body rebuild kit from autozone, EGR valve, MAP sensor, Distributor, Cap/rotor, IAC valve, Throttle Position Sensor, Coolant temp, Coolant sender, Knock sensor, Vacuum lines, Ignition Coil, Plugs/wires, O2 sensor<O</O
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Sense replacing all these parts, the truck runs great, and has tons of power, but it has this funky loping at idle. Another strange thing is that when the A/C is turned on it idles great. Any help or suggestions would be greatly appreciated on how to get this idle down to normal.

 

Im starting to wonder if this is a grounding problem. My wife drives the truck to work and she said when she uses the blinkers, the gauges go crazy. I have the main ground cable going to the front of the block, just next to the water pump. Then the firewall is grounded to the exhaust manifold, I think I need to move it to a transmission bolt. I cant remember its original location.. Anyone know the original ground cable locations on a 1994 silverado 350?

 

WOW, 267 people have viewed my post and not 1 response.. Thanks..

 

i get the same thing on my posts sometimes, about 200 views with little or no response. i had a similar problem with my truck when i hit a bump and my gauges went crazy, it was a loose connection on the dash cluster. on the loping, it seems odd that when a/c is on it smoothes out. after replacing those parts there really isn't a reason it should lope. i'm stumped as well. sorry

 

Hey, thanks for posting chevyboy1993. Well tonight I cleaned all the grounds.. I got a wire brush and cleaned the connection to the block, and to battery, I also cleaned the connections to the frame and block/firewall. Still has this loping/surging and if I dont give it gas it dies.. After it warms up it will stay running, but still has this loping/surging thing. I am wondering if the computer is acting stupid. It gives me Check Engine light codes for parts that are brand new. Doesnt make sense. I checked the wiring to these new parts and its in perfect condition. Also the BRAKE indicator light on the dash is flashing. Can someone please give me some advice on what to check out next?

 

I think all the work on the engine made the IAC valve "go out of calibration." Turning on the A/C causes the PCM to command higher engine speed which, in this case, stabilizes the idle. See if there's an adjustment screw to set the minimum throttle opening to keep it from stalling.

 

There is no adjustment screw on the IAC. However the needle which comes out of the middle of the IAC can be unscrewed and completely removed from the IAC. The trouble codes from the computer are:

 

There should be an adjustment screw to set the minimum throttle opening that's separate from the IAC valve. Anyway, the ECM can't/won't control the ignition timing with code 42 set. So, whatever ignition advance you set will stay constant. As I reread your post #1, I don't see how all that was necessary. Double-check the wiring, especially power and ground connections, and reference voltages to the sensors. Also, keep in mind that aftermarket parts tend to be defective out of the box more often than their warranty seems to imply.

 

Ok, I have the original ignition coil, but I dont have the original dizzy. The new dizzy is a remanufacture from parts store. The problem was the same with the old and new distributor. So I will double check all the wiring, and try the original ignition coil to see if it resolves the issue. What other suggestions do you have? EinST, I really appreciate your help.

 

This is what the IAC looks like on my truck. I dont have a adjustment screw.