8.1L spark knock

cwh · June 7, 2005

I know this issue has been beat to death but maybe some of you folks may be able to shed some light on my issue.

I have a 2001 GMC K2500 HD with the 8.1/allison combo. It had 31K on the odometer when I bought it serveral years ago and it now has 66k. I bought it to tow my '88 Mastercraft ski boat, but last spring I bought a 21' travel trailer, toy hauler style. The truck has had an intermittant miss at idle since I bought it. About a year or so ago I changed the fuel filter and switched to platinum 4 plugs. Last summer I had a really steep pull up a hill and my check engine light came on and went back out after I stopped pulling my trailer. While I have the trailer in tow the truck pings excessively. I notice almost no ping with no load. The mechanic that I took it to said that it is the MAF sensor but wouldn't change it out as the code cleared after 10 start-ups or something. I got tired of waiting and replace the MAF myself. I am running amsoil dual remote oil filter and their oiled air filter. I noticed that there was oil in the air tube last time I cleaned the filter so i did a light oiling and cleaned out the throttle body and intake plenum. I have access to a snap-on scanner from work and hooked up to the truck and took it for a drive. I noticed that under WOT I get up to 5 degrees of knock retard. Mind you this is with no trailer. I only assume that it will be much worse with a heavy load. I checked the fuel pressure with the vacuum line off and it is 58 psi. There are no other DTC's in memory or history and the only one I saw was P0101 MAF. Anyway, I know that there are a bunch of you out there that are much better at this diagnosis stuff than me.

Thanks in advance,

Chris Harding

Anchorage, Alaska

Butch 02 Denali · June 7, 2005

I would look to a clogged EGR valve Chris.

Then injectors, clean them!

Could be a clogged cat too.

Do a fuel pressure test while driving to be sure it doesn't drop below 55 psi or so.

mmmikkke · June 8, 2005

You should also remove the plugs and examine them to see if one out of the 8 looks different. Look for signs of preignition...or carbon buildup. That might clue you in on a clogged injector, or intake manifold leak. One pinging cylinder causes the computer to retard the spark for all 8.

Haulin' 8.1 · June 8, 2005

Chris I always get a timing retard at WOT also. I think it's part of the torque managment. I've done everything possible to eliminate it and it never even changed. But the truck runs beutifully so I quit worrying about it.

I would avoid using any Bosch platinum plugs, from experience they don't last for long on any high heat application such as towing. Almost any major brand of double platinum is better. Iridium plugs are a best choice and the current OEM.

Get the fuel injection flushed. That will make a big difference. Find somebody who uses the on vehicle kind, don't take your's apart..

Vernon

TieRod · June 8, 2005

Same hear, mine always pulled 5deg at WOT.

N900WM · June 8, 2005

Chris,

Your problem is the boat.

My truck is much happier towing a Ski Nautique.

cwh · June 16, 2005

Thanks for the replys. I went camping with the wife and kids for the last week and am back at work now. I went ahead and replaced the suspected intermittantly bad MAF sensor. The truck ran and drove better than ever. I was pulling our heavy toy hauler travel trailer and pulled some very big hills in 4th gear. I achieved some pretty horrible fuel mileage but at least I had minimal spark knock. I still have just a slight ping at times when the allison is looking for the right gear, but I'm going to change to different plugs and we'll see how it runs then. Anyway, thanks for the replys and happy towing.

Chris Harding

Anchorage, Alaska