A Question for All Mechanics

bcfitzsimons · October 22, 2002

Here's a question for CMNT and all other mechanics out there-

If a warm engine (and only warm) has a spark ping just at the start of acceleration from a full stop or just when you hit the gas to accelerate up a hill (and then it goes away), is that a sign of a mixture that's too lean?

Had a buddy ask me about this concerning his 6.0L Sierra. I rode around with him, and sure enough-2 or three pings only when he starts up from a complete stop, or when he presses on the gas harder to try to get up a hill. And it doesn't last-2 or 3 pings max are what I heard.

He's got 10000 miles on the thing. And he's going to try my Top End Cleaner bit this weekend. I told him I'd give a shout here and see what everyone says-

Thanks-Fitz.

SoMnDMAX · October 23, 2002

From my experience, a burst knock is caused by an overly aggressive spark timing curve. There's many variables that can cause this- volatility of the fuel being burned, cylinder pressures, fuel mixture, and the efficiency of the combustion chamber, among others. Fuel mixture can and does affect it as well, but not as much as the timing curve.

There really isn't any "easy" way to fix it with a computer controlled vehicle, besides reprogramming the ECU. He could try running a better grade of fuel, which should help to minimize the knock. Decarboning the engine will help too, by lowering cylinder pressure, and removing possible carbon "hot spots" that can cause detonation.

CMNTMXR81 · October 23, 2002

It can be a combination of things. It can be both running too lean and running too rich. Can be from too aggressive or too late of a timing curve. Is the EGR system turning on in these situations that you mention? EGR not turning on could cause a temporary lean factor that triggers some knock.

Do a search on here in the tech sections, I think I posted before (forget how long ago) about knock and what causes it both from air/fuel mixtures as well as carbon buildup/hotspots in the combustion chamber.

A higher grade of gas as noted by it's octane rating is simply the fuel's resistance to ignite when it is atomized. Hence, an 89 grade is slightly more resistant than an 87 at igniting, therefore controlling knock a little as it isn't pre-igniting from compression forces or those hotspots.

I think it may have been in relation to a fuel question.

Seeing as though this is a relatively new truck (10,000 miles) and a 6.0 (which hasn't been out long) with modern electronics and a high powered ignition system. I'd pretty much eliminate and timing issues as it is all PCM controlled. I'd also eliminate hotspots. I'd first take a look to see if the EGR is turning on/off.

bcfitzsimons · October 23, 2002

Hmm-well, he took it into his GMC dealer yesterday and apparently it needs an injector replaced and the MAF replaced too.

The injector I understand. But how the h*ll do you blow out an MAF? I GUARANTEE you this guy has never fiddled with his engine and/or MAF-he's got two left hands when it comes to working on engines.

Any thoughts? My guess is that a malfunctioning MAF will not signal a strong airflow into the throttle body, thereby signalling the PCM to inject a smaller (and thereby inappropriately lean) amount of gas from each fuel injector. But that's just a guess-from what he told me, the rest of the PCM checked out OK (and so did the timing curve, I'm then assuming).

Mind you, this is just an intellectual exercise on my part-thanks-Fitz.

Fitz.

Cop_Dodger · October 24, 2002

You are correct on the MAF sensor. I've seen them get trash in them and send out incorrect peramiters, normally rich. The MAF mesures air density. If it reads a smaller amout of air comming in it will react and add less fuel to the mixture causing a lean mixture which will cause spark knock or ping. Same thing happened to my 94 Mustang. I had to replace the MAF.

moscow_silverado · October 24, 2002

How do you check and see if the EGR is turning on/off?????

THanks ???

bcfitzsimons · October 24, 2002

Well, he got his truck back yesterday afternoon, and now he's really frustrated-because his dealer didn't fix the problem. I guess he's taking it back today to have them take a second look at it.

I'll let you know what he tells me as to what the dealer finds.

Fitz

MiSdIrEcTeD_1 · October 28, 2002

Hmm-well, he took it into his GMC dealer yesterday and apparently it needs an injector replaced and the MAF replaced too.

Replacing the MAF is their answer to everything... I had a guy actually tell me that because my MAF was bad it was what caused my tranny to burn up 2nd gear/band... I was like... "Do I look like an old lady or somethin to you???" Bunch of damned idiots... They had already replaced it when I went to pick up my truck, and wouldnt take the new MAF back since it was now "used." I ended up having to pay for it (to keep from destroying what credit I do have as an 18 year old) because removing the screen off of my original voided the warranty for a free replacement which I didnt want anyway...

bcfitzsimons · October 29, 2002

Well, my buddy tried my top end cleaner "universal fix-it" in his engine and it worked-no more warm spark knock.

Guess it was due to a deposit-and the thing has only 10000 miles on it. Go figure.

Fitz.

CMNTMXR81 · October 29, 2002

Also, those sensitive little resistors in the MAF either get coated with something that doesn't allow them to read correctly or they get broken from the wire posts.

In order to check if the EGR system is turning on/off, you need a program like A-Tap or a TechII scanner that can monitor the switching in light to mid throttle applications.

However, if you are outside the vehicle when it's running (usually rigvht after it's started) you can hear an electric solenoid motor turning on and off. That's your EGR pump. If you hear it, it's working.