Chevy 5.7 low speed surge

[Gene Arnold]

My son has a '94 Silverado 5. 7 gasser that used to be mine. Way back when it was mine (I bought it in '98 with 165,000 miles) it had a low speed surge (like around 1000 – 1200 rpms under load). My son's a pretty fair shade tree mechanic but tends to be so busy being a cop, a county supervisor, and a dad to four kid's whose vehicles often need him) that he seldom gets around to his own stuff. This surge was irritating to me and it is to him as well. Truck now has about 230,000 miles. Unrelated to the surge issue but so you'll know - during the last three years he's installed a new throttle body, a new fuel pump, a gas filter, and a thermostat. I'm giving a shout out to you guys before I book it in to a pro for diagnostics. Hope you can help. Any ideas?



Gene

 

[mwilson]

My son has a '94 Silverado 5. 7 gasser that used to be mine. Way back when it was mine (I bought it in '98 with 165,000 miles) it had a low speed surge (like around 1000 - 1200 rpms under load). My son's a pretty fair shade tree mechanic but tends to be so busy being a cop, a county supervisor, and a dad to four kid's whose vehicles often need him) that he seldom gets around to his own stuff. This surge was irritating to me and it is to him as well. Truck now has about 230,000 miles. Unrelated to the surge issue but so you'll know - during the last three years he's installed a new throttle body, a new fuel pump, a gas filter, and a thermostat. I'm giving a shout out to you guys before I book it in to a pro for diagnostics. Hope you can help. Any ideas?



Gene

The very first thing I check on that somewhat primitive fuel injection system is the coolant temp sensor. A decent scan tool will capture that reading. If the coolant temp of the engine is 180 degrees and the coolant temp sensor is telling the PCM that it is only 130 degrees for example then the PCM sends more fuel to the Throttle Body. It is only about $15. 00 and a bad one will really raise hell.

I had one so bad that the engine would start and run like a race car. After 5 minutes it would shut right down. Let it cool off and it would start again.

Finally got a scanner on it, the coolant temp sensor was indicating -40 below. Drowning in gasoline.....

Since then I make sure that the sensor gets checked when asked for advice. They don't always fail, but will get out of range.

Remember a 350 will suck quite a bit of excess fuel before it becomes obvious.



There are many other potential causes but check that sensor first. IMHO.



Mike.

 

[Wayne M.]

Very hard to diagnose from here. MIL on?

 

[Teich]

As Mike days get the system scanned, if no light is on it could be a vacume leak some where hose or intake, if a vacume leak it will be a small one. Laurie

 

[Wayne M.]

A small vacuum leak will be compensated by the IAC.

 

[Gene Arnold]

UPDATE:

I talked with the mechanic prior to taking the truck in for diagnostics. He suggested we disconnect the vacuum line to the EGR valve and plug it to see if it changed the surge. We did and my son reports a better performing engine. Now if he can find time to change the part out it looks like the problem is solved.

Thanks for your responses!