I've been using my AutoXray to try and determine why I sometimes have spark knock after starting the engine when it is warm. This is a 3.4 V6 in an '87 GT.

From observations, when the engine is cold, the idle spark is set to 30.1* while in open loop. When it goes to closed loop and warms up (normally a short driving distance or idle time), the basic timing will drop to 18.2*. When the base advance is at 30.1, max advance can go up to 51.+*. When it drops to 18.2, then the max drops to 39.9*.

So, I know that the base timing under the ECM is 30.1* or 18.2* depending on which flags are set from the various sensor readings.

Now this is where I have a problem. From a full cold start, everything seems to work fine, good power, no hesitation and no detonation. But if I shut down for a few minutes and restart, the base timing reverts to 30.1* and doesn't drop back to 18.2* after going closed loop. I sometimes drive for several minutes or miles before the cold advance is removed. Because of the advanced timing, I can experience spark know on hard accelleration but I still have a smooth power band with no other apparent ill affects.

All of the sensors are new and there are no codes. I have a 180* thermostat and 200* fan switch. The normal operating temperature is 185* with a 900 RPM idle. It runs good both cold or warm. Dyno show 139 RWHP with 190 pounds of torque.

I'm sure that there's a sequence of events that cause the 11.9* addition to the base timing. I know that a cold start in open loop is one and that a warm start does the same but the events to drop the timing don't seem to always be met in the time I think they should.

Maybe TK or one of the other ECM gurus can help as they have worked with the code and can follow the program logic.

I've tested with another ECM and chip and the readings are identical.

Thanks,

James

[This message has been edited by ammotrup (edited 05-02-2003).]

If cold advance is activated while in closed loop with a warm engine what is your rpm at idle during that condition? I have a different problem concerning high RPM at idle on a warm engine in closed loop and your unique condition may shed some needed light. I been thru the sensor and ECM routine also. Sorry I dont have an answer for your problem.

CLW

Quote--All of the sensors are new and there are no codes. I have a 180* thermostat and 200* fan switch. The normal operating temperature is 185* with a 900 RPM idle. It runs good both cold or warm. Dyno show 139 RWHP with 190 pounds of torque. ---Unquote

It is possible for a sensor to provide a defective output signal and not trigger any fault codes. Try a new coolant temperature sender and see what happens.

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87GT 3.4 Turbo Best 0-60 5.2 seconds
http://turbofiero.fierojoe.com/turbo.htm

Fiero86SE28,
The idle after a warm start is normal and starts at little higher but drops back to 900-1000 when the O2 cross counts start moving. The timing just stays up.

Dennis,
I stopped after work yesterday and got a new temp sensor. I haven't changed it yet but the current readings seem to follow the guage. The MAT and ECM water temp are the same (or very close) after sitting overnight.

Thanks,

James

Just a dumb question, are you driving around with the AutoXray hooked up to get these readings? If you are, your car is in diagnostic mode and you won't be getting correct readings. The manual for the AutoXray recomends that you don't drive the car while it is hooked up for this reason. You need a better scanner to be able to drive around and get accurate readings.

I've verified the readings using WinALDL and the results seem to be the same. I've been using the scanner because it's a lot more compact. I've been using the data monitor function which is a scan of the ALDL data stream.

I started the monitoring because I was only experiencing the spark knock after making a short stop with the engine off and then getting back up to speed in traffic.

I've taking some WinALDL captures to track the BLM since I put the new engine in last July(15,000 miles ago). Idle BLM runs 120 and otherwise it runs in the low 130s under cruise so I don't think it is running too lean. I quit playing with the fuel pressure as I didn't want to cause a rich condition at idle. On a recent trip to Columbia (220 miles mixed interstate and two lane), I averaged 31 MPG so I'm pleased.

James

What did you set your fuel pressure to and how? The instructions that came with my Holley fuel pressure regulator said to do it with the vacuum hose disconnected. I set mine to 44 psi and it seems to be good there. With the vacuum line hooked up the pressure reads around 38 psi at idle.

I installed a Hypertech fuel regulator. It has a 5/16 hex adjuster and with small hands and a string tied to the wrench (You're going to drop it and the lanyard makes it easy to retrieve - Fieros love to eat stuff like that.), you can reach the adjusting screw.

I set the static pressure to 48-49 lbs(engine off) and that's about 41 at idle. A quick open of the throttle from idle brings it up to 45-46 so at WOT it should be around the high forties.

The injectors are AC Delco for an '99 and later 3100. The same part also crosses to an after market number that shows for both 3100 and 3400. The flow is somewhere around 16+ I would guess but I haven't found anything to support this. I do know that they were way too big when I put them in my 2.8 before the 3.4 was installed.

I do have a bored throttle body and upper intake with Sprint headers on the GM crate motor.

I've got a set of 17# Accel pintle style injectors but haven't tried them yet. I read a lot about the fact that the pintle injectors are better matched to the ECM versus the disk style.

The cables used for WinALDL also put the car into diagnostic mode, I believe. It all depends on how it's wired.