Lean Misfires at 2700-2800 rpm

[DoctorDex]

Here is a brief summary of the 19 pages of posts in this thread:

If I am accelerating/cruising/revving in neutral, in the 2500 - 2900 rpm range, the car misses/hesitates, epecially @ 2600 rpm. I have made numerous logs of the issue and here is what I've found:

O2 Trim:
At 2450 rpm, the O2 trim starts to increase from normal (~100)
By 2600 rpm, the O2 trim hits 168 (max).
By 2800 rpm, the O2 trim starts to come back down
By 2950 rpm, the O2 trim is back to normal (~100)

If I add more fuel (14 - 17%) to bring the O2 trims back down to ~ 100, it still misses.

RPM & Timing:
If I rev the car in neutral, timing and RPM will plateau briefly once it gets to 2700 rpm. If I have the exhaust in sport mode, I'll hear a loud pop when it misses.

Knock:
While revving in neutral, more often than not, I'll see 1 - 2 counts of knock when it misses. During cruise, but more often during light accel, I'll occasionally see sustained knock counts of 1 - 2 counts from 2600 - 3000 rpm (total knock sum of ~ 10 - 12).

WOT:
Despite adding extra fuel, I'll still see a brief lean spike in the 2600 - 3000 rpm range when it misses. The lean spot, secondary to the miss, occurs simultaneously on the front and rear banks. The miss is less noticeable at WOT, but it is definitely still there.

Methanol:
1) Normally the rpm and timing plateau @ 2700 rpm when it misses. While spraying meth the car revs smoothly and there is no plateau in the rpm and timing.

2) Normally, while cruising @ 2700 rpm, the car misses constantly and the timing fluctuates +/- 1* at a fairly rapid pace. As soon as I start spraying meth the missing stops and the timing holds rock solid.

This information points to a fueling issue/air and not a spark issue.

Things I've tried to fix the problem:
1) Hotwired Supra fuel pump.
2) Disconnected stock FP relay
3) New PTE 580 cc injectors. Matched & flow tested x 2
4) Aftermarket FPR
5) Aftermarket Fuel filter & SS braided lines
6) New ECU (used) - known to be good.
7) New OEM coils
8) Wires x 3 (Accel, NGK & now MSD)
9) Plugs gapped to 0.028 x 4 sets (NGK Coppers, NGK non-resistors, NGK iridiums and now Denso IK27s)
10) GM MAF & MAFTransltor (No longer use stock MAF)
11) Checked resistance of CAS
12) Checked resistance of PTU
13) Checked Resistance of Fuel injector resistor pack.
14) Checked compression. All cylinders are between 142 - 149 psi
15) Sent oil away for analysis x 2. Everything checks out except iron was slightly higher than average (20 ppm).
16) Vac. line reduction & emissions delete
17) Disconnected TPS. no effect. subsequently reconnected.
18) New OEM ISC
19) Increased base timing to 10*. Made the miss _feel_ less noticeable. Definitely still there, no change in the logs.
20) HKS DLI - No effect on the missing.
21) New Optima Yellow Top battery
22) All Rubycon caps in ECU & ECS changed. Traces in the location of Rubycon caps verified.

Things that made the problem worse/more noticeable:
1) Lightweight aluminum flywheel.
2) GM MAF in blow through?

Things others have tried that I have not:
1) Changed PTU
2) Changed engine bottom end
3) Swapped fuel injector resistor pack

Strange things I've noticed:
1) While cruising @ ~ 2700 rpm it misses continuously. This is coupled with very slight pressure fluctations, (ie. +/- 1 cm Hg). However, once in a while there is an unusually large "miss" and the car lurges a bit and I'll see a relatively large manifold pressure spike, measured with an electronic boost gauge (ie. from -10cm Hg to 0.00 and then back to -10cm Hg). These pressure spikes are seen by the GM MAF as changes in air flow, ie. during the small pressure fluctuations AF readings oscillated +/- 30hz and during the large pressure spikes AF readings oscillated +/- 100Hz.

2) If I cold start the car and then rev it up in neutral, it revs smoothly through the entire rpm range. Once the car gets warm, ie. coolant temp of > 120 - 130* F it starts to miss. On the other hand, if I try and drive the car right away when it is still cold, it'll still miss in the 2500 - 2900 rpm range. In fact it misses horribly when I drive it when it is cold. At times it will literally fall on its face once it gets to 2600 rpm. I think this has to do with the ECU adding extra fuel while the car is warming up and this effectively masks the problem.

3) Hot start problem. Car has a hard time starting when hot. However, if I run the pump, for ~ 30 seconds using the check connector), it starts up easily, however, it stills wants to run extremely lean and rough (as if its missing). If I try and rev it up in neutral, I can't get the rpms above 2600 rpm because of the missing. Makes me wonder if there is something under the hood, that is sensitive to heat, causing the missing and/or lean condition. This issue probably isn't helped by the fact that I'm using a MAFTranslator that feeds a fixed temp signal and I have deleted the FPR solenoid.

4) Jumpy OEM tach in the 2500 - 3000 rpm range (esp. @ 2600) while accelerating. Also noticed the tach in logs looks jumpy @ WOT throughout the entire RPM range. Unaffected by installation of HKS DLI.

5) Occasional low load sustained knock while gently accelerating @ ~ 2600 rpm. Occurs with/without the EGR connected. See up to a 12 counts of sustained knock, which continues despite 7* of timing being pulled by the ECU. Seems to continue until I take my foot off the gas. Edit: I have later come to realize the vacuum nipple where my EGR was connected is plugged. Therefore, I suspect the EGR was never working properly.

7) Loud ticking from my rear valve cover, in the area around cylinder 2. I have 99 lifters, so it shouldn't be lifter tick. In addition, it sounds louder than lifter tick. I've sent my oil away for analysis and they've verified that it isn't rod knock.

8) When I drop my downpipe, my rear O2 housing is white (lean) and my front O2 housing is black with soot (rich).

9) When I rev the car in neutral, while keeping the throttle position constant, the rpms gradually increase until it gets to ~ 2700 rpm. Then it stumbles, the rpms drop to 2500 rpm and then the cycle repeats itself. If I give it enough gas, it'll eventually make it over the hiccup, until it reaches 3000 rpm and then the same thing happens again. It also wants to do it at 3500 rpm. Its only once I get over 4000 rpm that it revs gradually and smoothly.

Things I'm planning on trying:
1) Testing my ECU in a friends car
2) Tracking down the source of the ticking from my rear valve cover/changing lifters and/or pulling heads.
3) Using an oscilloscope to check the spark dwell.
4) Getting my hands on an oscilloscope that allows one to log pulses and go back and review them later.
5) Reconnecting the EGR system and ensuring it is working properly.


Any suggestions/ideas would be _greatly appreciated_.

[mehlosTT]

Do a search as I am sure there is a thread on this topic from 2004 year.
Before going to the AEM ems, mine did the same at that exact rpm under boost...stuttered at 2800 rpms under slight accel. can't recall why the stock ECU did that, but I also had the S-AFC in there at the time too, and the ecu was getting old at 100,000 mileage.

[DoctorDex]

I tried searching already and I came across a few threads, however, none of them were very helpful. One thread suggested adjusting the base timing to help solve the problem. I've tried this to no avail. The way I see it, a bad ECU is a diagnosis of exclusion and I'd like to exclude all other possibille causes before I run out and buy an AEM.

Declan

[DoctorDex]

Basically, what I'm looking to find out is if anybody else, who didn't have a bad ECU, has had similar symptoms.

Declan

[TurboBob]

is your MAF draw-thru or blow thru?

Stock PCV system?

How is the BOV configured?

If draw-thru do you have that Y shaped BOV silencer installed in the intake?

I am trying to figure if there is air bypassing the MAF somehow.

Bob

[DoctorDex]

I have a blow through system with the stock PCV system + Krankvents. I have both the front and the rear krankvents installed.

My BOV is located before the MAF and its configured to dump to the atmosphere.

I'm going to check for boost/vacuum leaks today and I'll let you know if I find anything.

Declan

[DoctorDex]

I found a few small boost/vacuum leaks and I fixed them. This didn't help the lean spot at 2800.

I didn't get the chance to test for exhaust leaks, but I think there must an exhaust leak on the front bank, before the O2 sensor, because according to the NBO2 sensors my front bank is always slightly leaner than my rear, but when I pulled my downpipe last month, the front was black (rich) whereas the rear was white (lean). This would also explain why I'm getting misfires in the 2200-2800 rpm range (closed loop), but not in the open loop 2200-2800 rpm range.

Declan

[keithmac]

Declan, could you post picks of your blow through setup please, interested in having a go at this myself.

[skunkworks]

I have the same problem running draw thru. I really have given up on it. (rough running in closed loop between 2500 and 3000) Wot thru these rpm ranges are just fine. I had worked it out a while ago by fiddling with the maft - (making the 3000 rpm range pretty rich) but have not been able to do it with my 560cc injectors.

Honestly - I am just buying time until I get the funds togather to buy the maft-pro.. I am really sick and tired of letting the stock ecu run closed loop. people running maft-pro "closed loop" with the wide-band have had really good luck with it.

sam

[DoctorDex]

Originally Posted by keithmac

Declan, could you post picks of your blow through setup please, interested in having a go at this myself.

I'll try and get some pics tonight.

Declan