Lean Misfires at 2700-2800 rpm

[venik]

the EGR system is the biggest one I think, and those are known to go out. get some EGR block offs and that will tell you if its the prob or not

[davidm5287]

The EGR system is already blocked off.

[MaG18]

hey guys! im having the same problem! i didnt wanna read through 13 pages but hes my thread: http://www.3si.org/forum/showthread....41#post4129741

could u guys tell me if its cause of a fedspec ECU on a calispec car? i was told by numerous ppl that it shouldnt matter. any input?

[TurboBob]

I just re-read all 13 pages........

What I did NOT find.............

Log data.

The ECU is doing something, perhaps decel fuel cut-off or enleanment (rescaled airflow for injector size for most of you).

Someone (everyone) needs to do some detailed logging of the threshold of this occurance. Look at all the trims, TPS, airflow, trims, injector pulsewidth, etc.

Watching fuel pressure is revealing also, especially when the stock resistor+relay is still on the car. These cars are not set up to easily monitor fuel pressure while driving, but for those with the means, please try it!

Something is happening there. The "open loop test" is a pretty good test to see if it can be changed. I may plug my O2's back in just to see if I can replicate the problem (1991 RT/TT, stock fuel system + bypassed resistor)

Bob

[DoctorDex]

Bob,

I apologize that I don't have any logs handy and my car is in storage at the moment so I won't be able to make any new ones, for a few months time, but I'll tell you what I remember from memory.

During the misfires:

TPS stayed constant
O2 trim (the instantaneous one) would bounce between ~160 and ~60
IDC/IPW I can't remember if it changed, but I'm pretty sure it remained relatively constant.
O2 sensor voltage would bouce back and forth between rich and lean.
Timing would bounce around +/- 5 degrees.

When it would start missing, if I kept it at the same rpm point eventually it would start to knock.

I'll make a log of it as soon as I get my car out of storage. Thanks for your help.

Declan

[TurboBob]

The O2 trim hitting 160 and then dropping is a huge indicator of whats going on.

The short term trim is running out of adjustment range, and the ECU is "defaulting" or something. Watch the O2 trim just before it starts to act up and see if it is climbing with RPM.

Basically, if it is what I think it is, "its your fuel map, its got a nasty hole"..... (to quote a cheesy movie)

forcing the ECU into open loop (disconnecting the O2 sensors) patches the hole.

Anyone else got logs......?

B

[davidm5287]

Originally Posted by TurboBob

Basically, if it is what I think it is, "its your fuel map, its got a nasty hole"..... (to quote a cheesy movie)

If that is the case, wouldn't swapping in a good known ECU fix this problem. The guys at ESP swapped in two of their working ECUs and the problem still occurred.

I will however try and get some logs either Tuesday or Wednesday, as soon as I can find time between school and work.

[iceman8446]

I'll have some time tommarow. I'll try to get some logs.

[DoctorDex]

Originally Posted by TurboBob

The O2 trim hitting 160 and then dropping is a huge indicator of whats going on.

The short term trim is running out of adjustment range, and the ECU is "defaulting" or something. Watch the O2 trim just before it starts to act up and see if it is climbing with RPM.

I'm sorry, I should have been more specific when I said it hits 160. I'll be cruising along, at an O2 trim between 80-100 and as soon as I get to the point(s) (I have 2 points now in the 2600 - 3200 rpm range), it misfires and the o2 trim spikes up to 160 and then back down between 80-100. If I stay at the same RPM point it'll do it constantly and the o2 trim will bounce back and forth.

I've tried richening up the mix, so that when I hit those points I'm cruising at an O2 trim of 60-70, but it still misfires and it still spikes up to 160 and then back down to 60.

Since the O2 trim spikes to 160 seem to happen regardless of how rich the tune is during closed loop, I have a feeling the spikes to 160 are a result of the misfire and not the cause, but then again, this is only a hunch. It seems like it would make sense though, with all that unspent air being dumped into the exhaust, the o2 sensors would read lean. I just can't think of what could be causing the problem. You'd think it would have to be something that partakes in the closed loop fuel adjustment, but what??

[keithmac]

Mines doing it around 2800 again, isn`t that in the region where the FPR switched from resistor mode to full voltage? When my caps were bad it felt a lot worse, instant o2 trim would bottom out (sure I`ve seen it in the 40`s?) but o2 voltage was still pegged high.

I`ve got fuel filter, walboro and hotwire for fitting at the weekend, if my problem dissapears I`ll post back and that`ll narrow it down slightly. A 15 year old pump and relay may not be operating to there full potential..

A lot of people on the UK GTO forum seem to have cured it with just a fuel filter change?.