Found a ignition problem on the Dyno.....Modified stock ECU

[Lil Truck]

The past 2 days I have been attempting to fix a stumble that occurs under full boost at 4500 and above.
See attachment.

Stock 109 41k miles
New cam and timing chain - 100 lb springs
55 lb injectors, with modified ECU
Power Logger
AFX Wide Band O2
Stock 41k MAF w/KN
Stock Coil pack & Module
Heated O2 sensor
New set of Autolite 23 @ .030
New set of GM 8mm plug wires
All sensors set to spec
HP fuel pump and hot wire set to 43lb no vac.
BB 2 step
VDO tach w Caspers mod
Caspers knock gauge
Alky kit single nozzle

Dyno run, 110 and alky.

History, this is an 87 GN modified for TSA class. For the last year it has been the additional modifications for street driving and has been running flawless on the street. At the track in 2nd gear I have been feeling a mild miss or stumble (no knock) it is 8th mile so it just gets into 3rd at the end of the run.

I thought it was just plugs and changed them. Idle was smoother and street driving crisp.

The attachment shows what it is doing from 4500 up to 5400 redline. It does not matter if it is at 20 lbs boost all the way to 30 lbs.

The Tachometer connection from the Dyno was attached individually to all 6 plug wires. Only 2 of the wire were consistently steady with spark even at idle. After the first day I changed out the spark plugs, spark plug wires, MAF, and the coil pack & module. All New GM Parts from NOS for GM and Kirban.

The first pull was much better than the previous day but still not gone. As the motor and parts got warmer the problem gets more noticeable. All 4 pulls on the second day were slightly better but the problem is still there? I disconnected the VDO tach, no change. Made I run low boost alky turned off, no change.

The only thing that shows on the Power Logger files is a loss of RPM when the stumble occurs, every sensor reads perfect.

I have 3 Coil pack and modules. The coils all test good cold or hot.

Can the Modules be tested?

Could the modified ECU be causing this?

I appreciate you input,
Thanks Steve

 

[TurboBuRick]

Does RPM flat line? Got a log file?

RL

 

[Lil Truck]

RPM is lazy but not flat line. I will post a log file tonight.

Thanks Rick

 

[earlbrown]

"New cam and timing chain - 100 lb springs"


That's what jumped out at me. Would this be a flat tappet cam and/or did you break it in with the 100# springs installed?

 

[Lil Truck]

"New cam and timing chain - 100 lb springs"


That's what jumped out at me. Would this be a flat tappet cam and/or did you break it in with the 100# springs installed?

The cam is flat tappet, Champion Irons & springs. They may be 110 on the seat?

 

[Lil Truck]

Does RPM flat line? Got a log file?

RL

Here is the data log at the highest boost, forgot to say the chip it a 6.1 TT w/O2 correction ON.

 

[TurboBuRick]

Damn, that log looks flawless. It's not miss firing.You would see big spikes in AFR.

Earl's prolly going in the right direction.

 

[TurboTdr3.8]

Earl's going in the right direction.

I second that also , if you have a valve not acting right it will feel like a miss and with that much boost that makes it worse .

 

[Pronto]

A weak valve spring?

 

[Lil Truck]

Thanks everyone, I will pull some springs and get them checked out.

I have not experienced this before, thought it must be electrical.

 

[earlbrown]

Put a dial on #3 exhaust pushrod... If it passes, check the other lobes. Shouldn't take more than an hour to see if you have 12 lobes.

 

[Lil Truck]

Thanks Earl, I will do it.

They were good about 1200 miles ago when I replaced the HGs, but you never know!

 

[earlbrown]

In this day and age, I never trust a flat tappet with less than 5 or 10 thousand on it... and I really don't trust them after that much either!

 

[Lil Truck]

Update!

I changed out the valve springs and added some seat pressure 120 lbs, open 300 lbs. Engine runs smooth as before.
Checked out cam lobes, all checked good.
Changed out push rods as they were at .060-.070 pre-load. Now they are .030-.040
Leak down test during valve spring change showed all cylinders at 92-90 @ 100psi cold.

Power is much better, 416-410 vs 373-340 .......... The hesitation from 4500-5000 is gone, but there is a definite power stumble at 4700 and again at 5000. It is stronger at 4700 loosing 30 hp then recovering. No backfire just a short loss of power?

ECM?
Crank pickup?
Cam Sensor?

Here is the PL file and Dyno Jet log.

 

[3.8L V-8 eater]

I had similar issues and did a laundry list of swapping parts...It would be worse some days than others and chased it since last fall...Turned out to be a bad wire on the CCCI module that plugs into the side of the ignition module...Just something to check...

 

[Lil Truck]

Thanks, I will check it out!

 

[TurboBob]

what is the battery voltage doing during a pull?

what cam?

Bob

 

[Lil Truck]

Voltage reading on the PL is 13.8 before boost, 13.2 during pull, then back to 13.8 after.

Cam is flat 212/212 @ .455 at valve, stock valve train

Could I possibly need to close the plug gap? Everything is perfect until I pass 4500.

 

[TurboBob]

its worth a try.

B

 

[Chuck Leeper]

End play on the crank is?