REAL Detonation.....can valve train cause this?


May 26, 2001
I have been getting bad detonation on my car for a couple years now.

I have tried everything to try and resolve it and I am now considering that it may actually be a mechanical problem.

I think I can trace the problem back to when I rebuilt the engine last time, adding the GN1 heads and a comp 212/212 cam.

since then I have tried replaceing everything electronic in the car to find the source of the detonation.

I know the detonation is real by the numerous headgaskets I have hanging on my wall.

here's what I have tried to fix it.

Swapped all the below parts with known good parts

Coil/spark mod.
injector harness

I also upgraded fuel pump to a walbro 340
and went to 55# injectors.. tried numeous chips.

I also pulled my entire wiring harness to make sure the grounds were good (this was an unreal job!!!...never again)

added and relocated grounds.

finally I got fed up with it all and installed F.A.S.T. this weekend...and guess what???

same problem!!!!

I even checked my timing with a timing light.... when the ECM was requesting 20* I saw 20* on the light. (dialed it into 20* and read the mark on the crank pully)

it detonates at about 4500rpm everytime!!!!!
I was running a 50/50 mix of 114 and 93 octane 15 psi of boost and 18 degrees of timing.... the A/F ratio was below 12 and pulling a few percent of fuel.

the knock started at 6psi of boost... and the air fuel ratio was very safe. this is not seem tuning related.

now I have to consider that this is a mechanical problem.

my question is this.

can weak springs or an improperly ground cam cause this?

I am really fed up because it looks like I am pushing out the passenger's side HG again!!!
Back in 1989 my 86 GN developed a very serious detonation problem to the point where detonation could be heard as a rattle. I had very little experience at that time and few turbo Regal owners had a clue. So I assembled a list of every conceivable item that might cause the problem from electrical to fuel. I would remove a part from my friends turbo Regal and install in on my car. I would cross it off the list. After replacing just about every part the one part that was not replaced was the Cam Sensor. The dealer replaced it under warranty and the problem was corrected. That is not to say that the Cam Sensor is your problem but who knows, it may be.
Did not see it on the list but a clogged fuel filter can lean it out.

Simple suggestion.

Cam sensor could not be adjusted right ...very tedious without the tool ...the Caspers cam adjust tool is awesome.

Good luck
hmmm..... now that my car is a bank to bank and not SFI I am not sure how the cam sensor will effect it.

something to think about though... thanks for the reply.

and BTW my cam sensor is adjusted right ...just forgot to mention that in my last post.
Its not being leaned out if the FAST is saying the A/F is 12:1 and its pulling fuel, so that rules out fuel filter, etc. Has it blown the same cylinder every time? How do the plugs look? You can unplug the cam sensor after you start the car, it will still run. Its worth a shot, but I kinda doubt its the problem. Looking at the plugs can tell you alot. You could possibly have a bad fuel injector, thats why I asked if it was the same cylinder blown every time. Or possibly a bad injector harness. Its very easy to get the wires pinched under the module bracket, BTDT.
It blows the center cylinders (3&4)

I have replaced the injectors from 009's to 55's....same problem

I think a new fuel pump/bigger injectors/new fuel filter/FAST ECM would rule out fuel problems.

and the WB02 seems to agree that it is not a lean condition..... unless it is isolated to a particular injector.

I will check the plugs again tonight to see if any look leaner than the others.

thanks for the suggestion
Just a thought to try and help....has the block ever had any deck work done?If so, between 3&4 may be different than the rest of it.I know you said you put on GN1 heads,were they new?Might want to have them checked for straightness.
Hopefully this might help.Keep digging at it,you'll find it.
I had this problem years ago when I had a BGC AFPR. Turns out it would show fp increase but it was not flowing the required volume. Went to another AFPR and problem was solved.
The wrong MAF sensor will detonate the motor.
Check to see you have the correct MAF sensor. Also swap FPR with known good one.

And didnt you inspect the motor after the teardown from the

You guys aren't reading his post. The A/F is safe, so its not lean. FAST ecm's have no maf sensor. He is getting detonation, so its most likely not a surface problem. If it were blowing hg's and doing so with no knock, then I'd suspect a deck problem. Check that injector harness, and those plugs. You are getting knock retard, correct? I.E., its pulling timing out?
No Chris I did not degree the cam.

this is one of my concerns....I also did not have the springs measured to see if they are the correct hieght/pressure. Althought they are the springs that comp suggested for my cam. (flat tappet hydraulic)

and that is my question.... can something in the valve train being "off" cause detonation.

I know that a cam that is off can cause the car to fall on its face on the big end or have a bad idle.....but can it cause my sysmptoms? (severe knock)

and yes I am sure it is knock

#1 it is audible
#2 the knock sensor is definatley picking it up and pulling timing
#3 I have blown HGs to prove it :)

also I had the heads checked last time I changed HGs and they were straight and are new and have never been cut. The block has only been squared so I doubt this is a compression issue.

so, has anyone heard of a cam being ground/ installed wrong, a spring being too weak/strong, or pushrods being too long/short or lifters being "bad" that could cause this problem?

I think I have covered almost all the bases on this one and the valve train is the only part I really have not troubleshot.

I need a cam expert here ASAP!! :D
You may have a bad boost gauge? A buddy of mine thought he was running 23-25lbs of boost but we replaced his boost gauge he was actually running 30. Just a thought. Good Luck
concerning F.A.S.T. where do you have your o2 sensor located? In my dad's car, in the stock location the wide band read a lot richer than the car actually was. In fact it got so lean it burned up the head a bit and fast was still taking fuel out on a commanded 11.2 A/F. On my car.. either my F.A.S.T. o2 sensor reads a little rich or my car just prefers AF of 11 to 1. If I set it up for anything leaner than 11.2 my EGTs are above 1700. If you have it in stock location you may want to set up for a commanded af of about 10.5 and try that.

Carbon buildup in the combustion chamber can cause hotpsots that will cause detonation.

I would look closely at the fuel system, filter, regulator, check for vacumn leaks, pull hose off and on regulator and make sure the pressure changes, etc. Also may want to put a gauge you can watch to make sure that pressure isn't falling off under boost and that you are gettng a 1 for 1 increase in FP. Also check you gauge... we had one on the rail that read 6 PSI more than there actually was and didn't realize it until we added a hood mounted that read 6 PSI less. Got a new one for rail and it matched hood.

Does detonation occur at full throttle or part throttle? Also check your egr valve a malfunctioning egr will cause detonation. Usually
at part throttle it will ping like a bastard. Cam could be over advanced butt if you installed the cam staight up meaning at 0
you are ok there.
If you dataloged any of these runs, the FAST will tell you how much psi you are running. You can then compare it to your gauge. On my FAST the ecm reads 2- 3 psi higher than the gauge. I don't know which one is right, I should probably find out! Any results on plug condition yet? I also agree with sully on the 02 sensor location, in the downpipe is far better from what I've read and seen. I don't think I've ever heard of weak springs causing detonation. Valve float yes, but not detonation. You said it was occcuring around 4500 rpm's, even marginal springs would hold that down. I think we can rule out mechanical noise, since it is blowing hg's. I think you either have 1 or 2 lean cylinders from a bad injector wire or something, or you have a sensor "reading" problem, like the 02 is not reading correctly, boost gauge, etc. Even if the cam was accidentally advanced a few degrees, with the commanded timing and boost you're trying to run with the octane you have you should be OK.
Boost gauge , FAST map readings and fuel pressure gauge increase all agree that the boost is actual.

this is definatley not a boost problem.

besides, the knock begins as low as 6 psi.

didn't have time to check the plugs last night
I will do that tonight.

I also plan on moving the WB02 to the DP 8" from the outlet.

keep in mind that this was a problem BEFORE FAST was installed.
I installed FAST to remedy this problem.

WICKED: this car has GN1 heads and they do not have EGR ports.

V6 BEAST: I will try your suggestion on the 10.5 to one airfuel ratio tonight. thanks!
Thanks MSDGN, that is what I wanted to know, if anyone has heard of something goofy like this happening.

I plan on pulling the intake and measuring the lift at each lifter this weekend to see if some might be going flat.
I know you have already heard it in three posts, but check your cam timing. I will bet money that it's too far advanced