Here's one that has me stumped, my 87 GN went from running great to crappy in one day. I have recently installed a TH downpipe and Tomka Cold air and was getting around 15 to 18 psi with some knock at full boost. The car has Thrasher chip and blue tops, 43TS plugs gapped to 35. I turned down the boost to keep it under 15 using the adj wastegate, hoping to get rid of the knock. After taking the car out for a sunday drive the knock is worse, as soon as the boost gets to 10 there is heavy audible knock and the red lights on the knock gauge lite up. If I keep it under 10 it seems to accel ok. Here is what I have done to try to find the problem. Replaced the blue tops and injector harness with known good stock parts, installed Thrasher chip for stock inj, replaced coil pack and module with one from known good car, installed maf from same car, substituted ecm from same car. Same problem no change. I checked the crank and cam sensor waveform with a labscope while driving, no dropouts waveform looks normal. It knocks so loudly that I am afraid I will do some damage to the motor if it continues. I am thinking timing chain or maybe broken valve spring? does this sound right to any of you. I am starting to get discouraged, don't know what to check next.
Lots of Knock
- Thread starter Buzzard
- Start date
TR Custom Parts
Mark Hueffman - Owner
- Joined
- May 25, 2001
No mention of fuel system upgrades or settings. Did you go from a stock air intake to the Tomka?
Russ Merritt
New Member
- Joined
- Aug 11, 2002
Maybe a wiped cam? EGR works ok? Is there a skip to the motor or does it run smooth? Did you think bad gas, maybe your gas station f$#@ up. 87 octane would kick it's a$$!! If it ran good after all the mods you did and then one day went to s$#@ then I would lean to the gas theory.
Maybe its an oversight, but there was No mention Fuel pump, adjustable fuel pressure regulator, Fuel pressure or octane in your post.
If you have a good fuel pump, an adjustable fuel pressure regulator set at 45 psi and fuel pressure rises 1 psi for every psi of boost, put in a gallon or two of race gas and see if the problem doesn't go away.
If you have a good fuel pump, an adjustable fuel pressure regulator set at 45 psi and fuel pressure rises 1 psi for every psi of boost, put in a gallon or two of race gas and see if the problem doesn't go away.
I am running the Racetronics fuel pump and hotwire kit. Tried a different gas station, switched from texaco to chevron. I have 42 psi fuel pressure at idle with vaccum line on and 50 psi when under 7 or 8 psi boost using the larger gm pressure regulator. I can't test the pump any higher because of the knock. There seems to be plenty of clearance around the downpipe to avoid anything hitting. Seems to have a rougher idle than It used to also, almost like a slight miss. Still stumped.
BTW, your downpipe can hit all it wants it's not going to cause audible knock.
The racetronix kit comes with a Walbro 340 does it not? If that's the FP you got with your hotwire, then dont worry about that.. but your fuel does seem odd, 42 PSI with your vacuum line on? it's supposed to be off when you test it at idle.
Your best bet is to get a billet adjustable fuelpressure regulator, they are very accurate and can handle up to 100 PSI.
The racetronix kit comes with a Walbro 340 does it not? If that's the FP you got with your hotwire, then dont worry about that.. but your fuel does seem odd, 42 PSI with your vacuum line on? it's supposed to be off when you test it at idle.
Your best bet is to get a billet adjustable fuelpressure regulator, they are very accurate and can handle up to 100 PSI.
Russ Merritt
New Member
- Joined
- Aug 11, 2002
If it's runnin ruff, you could have a bad cam. Do the compression test, and watch how the needle rises. If one of the exhaust lobes is gone that cyl will still have compression but the needle will rise wierd.
Originally posted by Buzzard
I am running the Racetronics fuel pump and hotwire kit. Tried a different gas station, switched from texaco to chevron. I have 42 psi fuel pressure at idle with vaccum line on and 50 psi when under 7 or 8 psi boost using the larger gm pressure regulator. I can't test the pump any higher because of the knock. There seems to be plenty of clearance around the downpipe to avoid anything hitting. Seems to have a rougher idle than It used to also, almost like a slight miss. Still stumped.
Your fuel pressure is too low! Thrasher chips need 45 psi line off! If you still get knock, empty your gas tank as much as possible and pour in the "good stuff" 112 or higher and see if your knock goes away.
Also, you need to see what the fuel pressure is doing when knock occurs. Just because it rises with boost up to 50psi doesn't mean it will keep rising!
My car drove me nuts with the same problem. The fuel pressure would rise with boost up to 55 psi at 1/2 throttle. Any more and pressure would fall off causing knock. You need to take your car for a ride and carefully take boost up to the point of knock and see what the fuel pressure guage does.
Also check TPS after installing the Thrasher for your bad idle problem! Check the simple things first.
Good Luck
Thanks for all the replys I will do a compression test on all cylinders and see what turns up. Planing to pull the valve covers and ck springs and visually see if all the rockers go up and down like they should. I don't think that a blown headgasket is the cause due the the fact that there is no combustion in the radator or coolant in the engine oil and coolant temp is 160 to 180 all of the time. Do you feel that the adjustable pressure regulators are better that the 237 gm regulator? Why would you test fuel pressure with the vaccum line off? Wouldn't that give you a false reading because you drive the car with the line on? Do these cars jump timing yet still run when the timing gears go bad? With 120000 miles on mine I will be changing it anyway.
blackbuick87
Senior Member
- Joined
- Mar 11, 2002
Actually his FP is too high. 42psi line on translates to ~ 48-50psi line off. The 237 on mine was at 48psi also after the hotwire kit. It is just too powerful for the regulator to hold back. Order an adj Accufab regulator, they are excellent. You could have rich knock with all that FP. Try race gas and get that pressure down and keep us posted
Found the Problem
Found the problem, a uneven vaccum reading led me to remove the valve covers looking for a broken valve spring. Didn't find a broken valve spring but I did find that #4 exhaust valve was not opening due to the flat cam lobe, it appears that #3 is also on its way out. So, I will be installing a new camshaft and timing chain, along with a new set of valve springs and seals. This should fix my problem and hopefully add some horsepower.
Found the problem, a uneven vaccum reading led me to remove the valve covers looking for a broken valve spring. Didn't find a broken valve spring but I did find that #4 exhaust valve was not opening due to the flat cam lobe, it appears that #3 is also on its way out. So, I will be installing a new camshaft and timing chain, along with a new set of valve springs and seals. This should fix my problem and hopefully add some horsepower.
