Detonation: Where to start?

[ntsqd]

The combo: '91 Sub TBI 350 700R 3.73 285/75/16
The caveat: This is the first computer controled engine I've had to fix. I'm also tone deaf so hearing very mild detonation hasn't happened in the past.
The complaint: Has recently started detonating on fuel it used to be happy with. No obvious symptoms of a vacuum leak. Idle speed is about right. No whistling at idle. No real loss of power. Going to WOT sometimes cures it, sometimes not immediately - fades away. Downshifting ditto, but usually does. Don't yet have the info needed to pull codes, if any.
Some recent history: Chasing the Vegas to Reno race it developed a misfire under load. In the Circus-Circus parking lot in Reno we threw plugs (.035"), wires, cap, rotor, & fuel filter at it. Misfire was coil wire leaking thru a rub'd thru point. Had Bosch 4 ground electrode plugs in it (eewww!), now has 'normal' AC's.
The obvious: unplug wire & check base timing; will happen this afternoon.

What else?

 

[ryan22re]

Carbon build up in the cylinders? TBI motors were aroung 9.3:1 IIRC. Perhaps needs to be cleaned out, not just romping it, but maybe a little GM top engine cleaner.

But yeah, check the timing first.

 

[1979jimmy350]

has your check engine light come on yet? It is very easy to check the codes on theses older trucks
Use a paper clip on the two upper right terminals of the aldl conector, jumper these two together and then turn the truck to the run postion, your check engine light will start blinking then. First it will blink pause blink blink long pause then repeate, this is code 12. It will throw code 12 3 time then move on to any other codes displaying each 3 times.

Have you tried using a higher octane fuel to see if it cures the problem? You could have carbon build up in the combustion chamber raising your compression ratio
It could also be your ESC system, on the TBI engines the knock sensor when it dectects knock retards the timing to stop the knock. so if thats not working correctly that could be a problem

 

[ntsqd]

Thanks for the replies. No check engine light. So far it has only done it at the tail end of the last tank and this tank. I need tol fill it today anyway so I'll do so with higher grade & see what happens.

 

[loafer]

If it is only doing it when the tank is low I would suspect a fuel delivery problem. It it's leaning out from a decrease in fuel pressure you can get detonation.

Thanks for the replies. No check engine light. So far it has only done it at the tail end of the last tank and this tank. I need tol fill it today anyway so I'll do so with higher grade & see what happens.

 

[ntsqd]

Hadn't noticed that trend, but I'll now watch for it. Thanks.

 

[btolenti]

.....fuel pump going bad....bad fuel pressure?

 

[ntsqd]

Filled it with 91 b4 going out to the desert race. Still does it, but not as much. Today was 'work on the DD' day so I've not yet had time to pull any codes.

 

[beater_k20]

might also try some Seafoam. i've had that stuff cure all kinds of crazy things.

 

[K5dreamer]

detonation is caused by excessivly high cylender temperatures.

the most common cause of this is a failed EGR system or valve. exhaust gas is inert and despite being 1000 degrees or so, it actually keeps the temp of combustion down and avoids detonation. I would check your egr valve for function and the rest of the system as well. or have it checked.

first things first though, i would suggest an induction system cleaning done by a competent service station, throttle body cleaning, and a can of BG 44k in the gas tank. did that for my 89 IROC and it made a world of difference, matter of fact i used three cans of throttle body cleaner to get all the gunk out of my throttle body. but she ran much better afterwords.

other than that, look at your coolant temp at operating temp, if its high (230+) then i would suggest a lower temp thermostat. generally it means higher emissions but it will run cooler and help stave off detonation and these trucks dont really have emissions anyway. (depending on year, dont remember the year of the truck in question)

another possibility is your spark plugs, make sure you are using the recomended size and heat range the factory suggests with a TBI setup. hotter plugs will retain enough heat to act like a glow plug and cause detonation.

hope that helps a little

 

[FWP]

Got a laptop with a serial port? Take a look at the winaldl data, might help t-shoot the problem.

For example- as K5dreamer mentioned temp issues, if the CTS is bad- ie reading cold/ hot to the computer- the fuel amount/ timing will be wrong.