1994 Sub when cold has loud knock

[dk_jen]

Greetings all

I need some help and direction on a problem with my 1994 GMC Sub – 350 / 5.7 TBI, 2 wheel drive with 102600 miles on the engine. It has been my ‘pleasure’ vehicle for some years since I can’t afford to drive it to work every day. I change the oil every 3000 miles – about twice a year. I use it mostly to haul a trailer or the family around (sometimes both at the same time) and try to drive it at least once a week.

Anyway – late in August I was taking my youngest to college. As I pulled off the freeway it seemed to have a lack of power and rough idle. It also had a slight ‘knock’ if I accelerated easily and when I let off the throttle after accelerating it knocked a bit. It was like the ignition knock when I have the big trailer behind it. Drove home, had no choice, and it would knock when I pushed the throttle to go up a hill and again as I crested the hill and backed off. If I pushed it hard it would down shift to 3rd and run pretty good.

Next day I went off to see what I could see. Found a shorting plug wire #7 and replaced it and figured that was that. Next weekend I was off to town and as soon as I pushed it at all it knocked like something was coming out the bottom of the motor. Since then I find it knocks loud until the motor warms up then it is mostly the ignition type knock – not near as loud and only on accel or decel.

For the next month as time permitted I have done the following based on threads I have read here:

Checked the EGR -- just fine
Put in a bottle of CD2 detergent (sticking valves)
2 bottles of fuel system cleaner
Found and swapped out another bad plug wire on #6– no fire at all
Changed out the plugs, wires, cap, and rotor. Plugs in cylinders 2,4 and 6 were black and dry, others looked tan and clean.
No water in the oil and no oil in the water.

Last weekend I did a static compression test and found cylinders 4 and 6 have little compression – about 30 lbs. The other five all have 180 lbs. I added a bit of oil to 4 and 6 and the compression came up 15-20 lbs. It also seemed to blow back though the throttle body when testing 4 and 6.

SO - Before I swap out the head – I am assuming stuck/broke valves or blown head gasket between 4 and 6 – would the lack of compression cause the loud cold knock? I don’t want to fix the top end only to find I still have rod bearings going. Local shop says to put in a re-man motor is about $3500. I would just as soon put that into a new truck but can’t afford either right now.

Thanks in advance for any help you can provide

[mustangmike351c]

since you dont drive it alot you may have a sticking valve in 4 & 6 check your knock sensor and run number 5 & 7 plug wires as far apart as you can

[maxwedge]

This could also be caused by a bad cam lobe or something in the valve train not actuating the valves properly, pull the valve cover and watch closely, as the engine is cranked, at the rockers in the suspect cylinder, try and compare the movement to the adjacent normally operating cylinder.

[dk_jen]

Thanks - I forgot I did change the knock sensor as well. One of the first things I did. I will pull the valve cover and watch the movement. I still wonder if the low compression causes the loud knock when the engine is cold.

[dk_jen]

Well - the truck is running just fine now. I had a blown head gasket between cyls 4 & 6. Took me a couple of days over a couple of weeks but it is all back together and the knock is gone. driven it almost 200 miles since the repair. I guess the knock was from the rod or piston skirt when there was no 'load' on the cyclinder.

[JaVeRo]

Number 4 cylinder was firing as the number 6 cylinder was loaded with fuel, closing the intake valve and ready to begin the compression stroke.

That's some good detective work you did.

I have been studying the same problem on a massey ferguson 165. I was at a loss until my brother-in-law got me pointed in the right direction. I'll pull the head this weekend and see if he is right.

James

[ChevyWonTon]

I would agree - excellent diagnosis work. I'll bet you didn't feel that way as you were changing plugs, wires, cap, and rotor only to have them not solve the problem.

I am reminded that we should always examine the clues with whatever tools we have available before making any judgements. I have a compression tester myself but rarely use it because it's so darned hard to even see the spark plugs these days, let alone get that dang tester down in the hole.

Good work, and thanks for letting us know how it turned out!