Hey guys,

I have an 88 5.7 that has recently began acting quite strangely recently. When I first start the car cold, it will start right up, but after the idle settles down a little, the idle speed will bounce between about 400 and 1200 rpm. This will happen endlessly until I start to drive the car. Under acceleration above 1000 rpms, the car runs really well, but when I lift off to change gears (t5 swap a few years ago) or let off to come to a stop, the rpms plummet to almost 250 rpm, then slowly the idle will struggle back up to normal (700ish) after about 10 seconds and then it will crash down again and repeat the cycle endlessly, similar to the startup idle, just a different rev range.

Vacuum lines are ALL replaced yesterday, iac cleaned and idle reset by the famous reset procedure found on this website

 

Continued...

Sorry if everyone saw this post a few times, my phone totally failed me...

but anyways, no codes either, i'm a little stumped and very frustrated, any help is greatly appreciated!!!!

 

Sounds like a vacuum leak. Go back and check all of your line connections again. Make sure you check the one under the intake plenum just behind the throttle body. It's a little hard to get connected and can come loose pretty easy when you are working in that area. Good luck.

Steve

 

Also, check that your runner bolts are all tight. They tend to loosen up over time.