I have an 87 350 TPI that's stock. I've tried everything to get rid of the spark knock. At spec 6 degrees timing it's severe. At about 2 after TDC it's OK. I have replaced distributor, ignition control module, cap, rotor, cooler plugs, MAF (relays are good), IAC, knock sensor (working OK), O2 sensor (single wire, faults when disconnected and bench tests OK). The TPS is set to 5.4v. Fuel pressure with key on, not running is 44psi and holds, then lowers to about 40 running at idle. Car runs like a bat out of hell and idles perfectly smooth at 600rpm, but under load I can't get rid of the knock.

I just removed the intake to fix an oil leak and found one of the hard vacuum lines crushed. Even that fix didn't change the knock problem. I also cleaned and tested the EGR and it's solenoid, both working well. Cleaned and tested all 8 injectors.

I understand running regular gas will cause some knock, but it shouldn't make it undriveable at spec timing.

Any ideas?

 

What's telling you you're getting knock? Is it actually audible? Are you running a datalogger that's telling you knock is occurring?

 

Very audible when under load only. I don't have a scanner for OBD1 cars.

 

So you had this condition before and after pulling the distributor to do the intake? And for clarification, you're setting timing with EST bypassed?
(check your decimal point in TPS value you wrote).

 

Check TDC and the marks on the damper/pointer. The outer ring may have slipped and that would mess up your ability to correctly set the base timing.

 

Interesting thought about the timing marks.

This problem was there before the intake remove/repairs and there is no change. Setting timing with EST disconnected. Yes, you are right about the TPS. It's .54v

 

What octane fuel are you running?

 

Regular 89 octane. I understand that will make a difference, but not enough to make it un-drivable. I tried a full tank of 91 and no change.

 

How do you know the car is stock?

What heat range spark plugs are you running?

What HEI module are you running (aftermarket ones can actually create significant offsets in the commanded spark timing).

 

Doesn't the ECM control the timing once the car has started? I think it does.

 

It does, but the HEI module can actually create an offset such that (in the case of my Pertronix module), it was retarding the timing by 5° across the board. There's a table in the calibration regarding spark latency, which is what deals with the module's characteristics.

https://www.thirdgen.org/forums/diy-...ncy-table.html

 

So...it can only retard timing, then. Not visualizing how it could "anticipate" a signal from the ECM, and trigger the coil on it's own?

 

In post 26, RBob said he tested a couple and one of them resulted in advanced the timing by a couple of deg.

https://www.thirdgen.org/forums/diy-...ml#post3775917

 

Huh. That sounds incredible. The ICM can anticipate a signal that hasn't been generated yet. How?

Your link took me to post #1. No Rbob there. I couldn't find the post that you must be referring to.

 

Scroll down to post 26

 

I don't see any explanation in or around that post as to how an ICM can anticipate a signal that hasn't been sent to it, yet.

 

I'm not sure about the mechanism you're describing... all im saying is that he determined empirically that some modules can retard and some can advance.


I pulled this out of his post.

 

I'm not sure either...that's why I'm asking. I don't think that "mechanism" exists....or could.

Once the car starts, the ECM takes over timing. The ICM is a slave, a device that grounds the coil, then breaks that ground to create a spark -functionally, it's just like points used to do. The ICM get's the "Now-now-now-now...." (aka, the "timing") from the ECM when the engine's running, and it executes the grounding and breaking of the coils primary circuit. SO, that being the case, it's hard to understand how the ICM can interpret and modify the timing. Now, MAYbe, there is circuitry inside the ICM, that can delay (retard timing) the timed signal from the ECM.....though it makes zero sense since GM could retard timing all they want to w/in the ECM, but MAYbe. But even if that were the case....how can the ICM anticipate a signal it has not yet gotten, and go ahead and break the coil's primary circuit before (advancing timing) it's been told to?? A signal that has not yet been created or delivered to the ICM, by the ECM, yet. The "mechanism that I'm describing"....doesn't exist, -that's the whole point....so IDK how it's possible for the ICM to change timing -especially advancing it.

 

Not sure.. but when I ran that same test on my pertronix module, it measured a 5 deg retard offset from idle all the way up to 4000 rpm...

 

Copy. I'd love to understand how that works. I can see a delay (retard) circuit in there....but I can't fathom a way for it to advance.

How'd you measure to "see" the retard occurring?

 

Further down in that trail, I describe the tests I ran

 

I agree with checking the damper/pointer to confirm it is correctly indicating TDC. It's not uncommon for the outer ring on the damper to slip and rotate relative to the crankshaft.

I also agree with the question regarding whether or not the engine is stock. Unless you know all of the history of the car, don't assume it is.

Have the injectors been replaced and what are they? If they are original, they are shot by now and should be changed. If they've been changed, need to figure out what they are and what flow rate.

Check the casting numbers on the block (behind the driver's side cylinder head, where the trans bellhousing connects) to see what block is in it. Also pull the valve covers and get the casting numbers off of the cylinder heads to see what they are. The engine internals are who knows.

My '89 GTA with a 350TPI had audible spark knock under heavy load for a long time and tried most of the things you have done. It would start knocking on the 2-3 shift and continue while pulling 3rd gear. Everything was set up "stock". After a number of years, never really figuring it out, the headgasket let go. Tearing the motor down, I found it was in fact a 350, but not original to the car and had been rebuilt 0.040 over with stock replacement internals, stock spec camshaft. However, the cylinder heads were swirl port style from an L05 TBI truck or Caprice. That was more than likely my knocking issue. Swirl port heads have a faster burn than the TPI heads and require less ignition timing advance.

Moral of the story is to not assume your motor is stock, unless you know for sure.

 

How long have you had the car? From when the problem started have you run multiple tanks of gas through it from different gas stations? Reason I am asking you may have received a bad/contaminated batch of gas.
It also may be caused by a mechanical problem (a cylinder that has a bunch of carbon built up, a wrist pin/piston problem, etc).

 

I have always thought of factory timing specs as a starting point. I start there and then try to find what the car likes. That can change when other things change, like gear ratios changes.

 

I agree that factory base timing spec is a starting point. To get rid of the spark knock I've had to go to about 2 after TDC. That seems excessive when spec is 6 after.

 

I've had the car since February 2024, so many tanks of gas. I've also put a few cans of seafoam through the gas over that time. Everything seemed pretty clean at least what I could see when I removed the intake. I also cleaned all the carbon buildup off the upper and lower intake.

 

Check TDC.

 

How bad was the carbon buildup in the intake ports? If a lot then it also could be in around the valves and combustion bowl of the head. I recently bought an 89 Formula 350 and one of the previous owners routed the egr directly to manifold vacuum instead of the egr control valve. So it was getting egr anytime the manifold had high vacuum!

 

Gotta admit, mine ran full EGR for a while as well because the vacuum solenoid was broken. The intake tubes were solid black when I cleaned them.

 

I had an exhaust leak caused by broken studs that hold Y pipe to exhaust manifolds. I went to remove the exhaust manifolds to repair and the right rear manifold bolt broke off flush with the head. So now I have to remove the intake manifold and right side head to extract the broken bolt. I wonder how much carbon build up I may find. I was surprised when I pulled the Y pipe down that the inlet of the converter showed no carbon buildup and could shine a light through (no sign of any blockages)

 

Two things that may be helpful:
First, carbon shouldn't accumulate over time; carbon should coat parts initially, then the accumulation should occur at the same rate as the burn-off. Should. If carbon is accumulating progressively, there is something else wrong that is causing that to happen.

Second, you don't need to pull the head to get that stud out, if you have some resources, you can weld a nut on the broken bolt, and thread the bolt out.

 

My car has AC and it’s the very back bolt by the firewall. There is approx. 1 1/2 inches to work in between head and the AC housing. Since I only had the car a few months I need to pull the intake anyway to replace gaskets and have my injectors cleaned and tested.

 

1.5" from the head, huh? It's amazing that the manifold fits in 1.5"...and you could get a bolt in/out.

Well? As you were, then. I'd weld that b!tch and get it out.