Detonation and backfire through intake

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Oct 17, 2001 | 12:34 AM

Hendej

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From: Crystal Lake, IL USA

Detonation and backfire through intake

After downloading Craig Moates software and wiring up a $2.00 laptop interface, I made some logs. Everytime I go to WOT knock counts go up a couple and 4 degrees of timing are removed. Even happens flooring it in 4th from 1000 RPM. Base timing is at 6 degrees, 92 octane gas, all ignition parts are 1 month old, rapidfire plugs.

Also, only in 2nd gear, only at 3000-3500 RPM, only at WOT, I hear a single gasp from the intake, then it recovers and takes right off again, pulling hard. It happens so quickly that I haven't been able to catch anything while logging.

Are these related? And here's a really stupid question. Thinking back to vacuum advance distributors, flooring it drops the vacuum causing the timing to advance? Retard? Which one? The scanner says my timing stays the same going into WOT then increases with RPM. Is the ECM doing things differently than the old distributors used to?

Oct 17, 2001 | 01:07 AM

AS84

Melting Slicks

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From: Springfield MO

Re: Detonation and backfire through intake (Hendej)

Do you have any valve train mods? I had this problem before I changed cams. Whenever I put a little load on the car such as going up a hill in a high gear or mashing the excelerator my car would backfire through the intake. I had 86000 miles whenever I added the cam and did the head work and the was a bit of carbon on the valves that could have prevented them from closing all of the way. Anyway I reworked my valve train components by replacing the valves, springs, retainers, locks, rocker arms, studs, and push rods. I did this because I had it all apart for the cam change and it was just preventative maintence. Now I have no backfires.
What's your timing advancing to?
What is Craig Moates software?

Oct 17, 2001 | 01:47 AM

Hendej

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From: Crystal Lake, IL USA

Re: Detonation and backfire through intake (AS84)

No mods. Maybe it's asking for a valve job? Hope not, it is running very strong otherwise (butt dyno). I'm looking at some graphs of logs. Timing is 29 deg before WOT then drops to 22 and climbs back to 30 deg as RPM's build. Let off throttle and it jumps to 40. Inj PW climbs quickly to 8 at WOt then slowly to 10 as RPMs build. O2 slams high and stays there.

On a graph of knock counts vs RPM - 1st gear, none - 2nd gear 2 cnts between 2800 and 3000, then 2 more between 3000 and 3200, then no more 'cause 6 degrees is gone - 3rd gear 1 cnt at 2800 and 1 cnt at 3200 and the retard is going from 3 to 4 to 3 to 4...

Craig Moates software is a great little free OBD1 scanning and logging utility:
http://members.tripod.com/moatesenheimer/gmecm/

It works with pretty much any of these interfaces:
http://www.andywhittaker.com/ecu/aldl_hardware.htm

Oct 17, 2001 | 05:20 AM

AquaMetallic94LT1

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From: Hillsboro OR

Re: Detonation and backfire through intake (Hendej)

The spark advance requirements of a computer controlled engine is the same as the requirements of a mechanical/vacuum spark advance controlled engine. The computer system uses a table to set the spark advance for a given rpm and engine load. The mechanical advance system generates a straight line advance curve that increases with rpm and vacuum advance adds additional advance as the engine load decreases.

Most engines will tolerate lower octane fuel using an advance curve that starts out at high advance value at low rpm (say 20 degrees at 1000 rpm ) and then dips down to a lower advance value at mid rpm (say 15 degrees at 2200) and then rises with rpm until it reaches some maximum advance level. The mechanical advance system cannot generate this kind of curve. The computer can also adjust the spark advance when spark knock is detected.

You mentioned that the O2 sensor value goes high. I think you have a lean condition at WOT that is causing the detonation and backfire. Possible sources are malfuntioning EGR system, plugged catalytic converter, clogged fuel fitler, weak fuel pump and bad injectors.

Oct 17, 2001 | 12:59 PM

Hendej

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From: Crystal Lake, IL USA

Re: Detonation and backfire through intake (AquaMetallic94LT1)

Aqua,

I'm looking at a graph of LV8 and O2 millivolts versus time over a 6 minute logging session. At low load (LV8 50-75), O2 ranges from 200 to 800, very busy. Every time I nail it, LV8 goes to 175-225 and O2 goes to 900 for the duration. I let off and LV8 drops to 25 while O2 goes to zero. That's where decel enleanment should be happening and that's why I assumed zero O2 volts were lean. So it's the opposite?

Oct 17, 2001 | 01:17 PM

jfb

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From: Cincinnati, Oh USA

Re: Detonation and backfire through intake (Hendej)

In a mechanical distributor, the vacuum advance pulls on the points and advances the timing (BTDC) under high vacuum. Light loads cause high vacuum and the fuel/air mixture burns slowly and therefore must start sooner. Heavy loads and WOT causes low manifold vacuum and the vacuum advance retards the spark timing because the fuel/air mixture burns faster and must start later.

Oct 17, 2001 | 01:57 PM

Hendej

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From: Crystal Lake, IL USA

Re: Detonation and backfire through intake (jfb)

Ok, now I remember pulling and plugging the vacuum line to set base timing - more vacuum = less load = advance
less vacuum = more load = retard

Got it!