'99 Yukon Rough Idle and Unknown Noise

[McDermut]

Good evening everyone!

I'm having issues trying to diagnose a rough idle and unknown engine noise I've been experiencing. Over the past six months, I've replaced the spark plugs, plug wires, cap and rotor with correct AC Delco parts but the issues are still persisting.

I've made an unlisted YouTube video linked below, but from 0:50 onward is when you can really see and hear what I'm talking about. I rested my phone on top of the fan shroud.

(Headphones highly recommended to hear the noise.)

I have some of the RPO codes for my vehicle below, but if y'all need more info feel free to ask!

Any comments and advice is greatly appreciated!

 

[exp500]

Have you adjusted the valves, checked for vacuum leaks, replaced the spider?

 

[McDermut]

Have you adjusted the valves, checked for vacuum leaks, replaced the spider?

Haven't adjusted the valves or replaced the spider yet, or for that matter anything that requires removing valve covers or the intake manifold. I haven't noticed any vacuum leaks but I've not actively looked, where are they typically on the 5.7?

 

[exp500]

If I tell you where to look you will ignore other possibilities. Wherever there is vacuum.

 

[OR VietVet]

Use a mechanic's stethoscope and remove the metal probe from the end and use the hollow flexible tube to listen for vacuum leaks. The spider can be replaced without removing the intake or valve covers. The spider is well known as a source for running problems.

 

[YukonGTmaster]

Good evening everyone!

I'm having issues trying to diagnose a rough idle and unknown engine noise I've been experiencing. Over the past six months, I've replaced the spark plugs, plug wires, cap and rotor with correct AC Delco parts but the issues are still persisting.

I've made an unlisted YouTube video linked below, but from 0:50 onward is when you can really see and hear what I'm talking about. I rested my phone on top of the fan shroud.

(Headphones highly recommended to hear the noise.)

I have some of the RPO codes for my vehicle below, but if y'all need more info feel free to ask!

Any comments and advice is greatly appreciated!

I had a rough idle, stumbling, and a high pitched squeal which I thought was a vacuum leak. I did the Felpro intake manifold gasket kit and MPFI Spider upgrade. Problem solved.

 

[McDermut]

Thank you all for the advice and information so far. I wanted to make an update on the issue which I haven't been able to fix yet due to mostly college mayhem and a more pressing head gasket leak onto the block. I have also been gathering more information to try and narrow down the issue(s) better and not spend where it's not necessary.

One observation I've made is that the shuddering and noise happened much more often and was more noticeable/audible during winter months when it was fairly cold or colder. Contrastingly, the issue is less frequent during the Spring/Summer months when it's typically 85+, and even then it's a less audible noise that lasts about five or ten minutes into driving (when cold it has lasted up to 15 minutes).

I also conducted a little experiment on two separate occasions a couple of months apart to determine approximate highway MPG, which came to 16 (both times it the math resulted in 15.8 or 15.9 that went beyond the thousandths and easily rounded up to 16) Both instances were trips of 100+ miles highway speeds, cruise control and very limited slowing or stops. Gas was the same brand and 87 octane. I'm in the process of calculating city MPG.

I hope some of this info can be useful, will be posting more info as I can.

 

[exp500]

If you are loosing coolant, replace lower intake manifold gaskets.

 

[tom3]

Do I see an electric EGR valve on the front of that intake? Pull that off and take a close look at the pintle and seat for a small chunk of carbon holding it open just a bit. Look real close.