TBSS Surging bucking just below 3K RPM: Stock

[WarWagon]

I have a stock 6.0L Trailblazer SS. Since last summer I get a bucking/surge when I take off from a cold start. The engine can be warm from the morning run and still surge. I think it could be a cold / open loop issue.

I had it tuned and it eliminated the surge until the weather cooled off.

Dealer is no help. When they get it out of their parking lot the issue is over with. No DTC's.

The surge is the worst just before 3K RPM then it quits. (2500-3000RPM) This is light throttle as mashing the throttle to the floor does not surge. Power braking while at the light throttle and keeping it under 3K RPM makes the bucking worse. You can add throttle to a point and add to the surge. This also happens in 2nd gear. After about 2 city blocks this issue is gone until the next startup.

I attach a log of a light surge just before 3K. I think it is going lean just below 3K RPM from the O2 Sensor readings. It is rich and then suddenly goes low voltage and then back to 800+ for readings as RPM goes up.

This issue is the reason I got HP tuners. This is my first post. Ideas on what to log and how to go about solving this are welcome. Any parts that are borderline not working like O2 sensors? Even if I just capture the problem area and hire a tuner to fix it...

VCM Version 2.1.18.0

The pre tune history of this is here:
http://www.trailblazerss.com/forums/...showtopic=9498

[Russ K]

Could you post your stock tune? I see that the LTFT's are 0, but the AFR is lean as the STFT's are up to 20% at frame 573. No wonder you have a hesitation/surge until closed loop. Any chance that your Maf needs to be cleaned? Do you have an oil type air filter installed?

Russ Kemp

[BBA]

Have you cleaned your throttle body? That's the most common cause for a cold engine surge.

[BBA]

I just read your thread...you already did the TB cleaning. Send me your email and I will send you my tune which I know works perfect, plus you will like it better than mail order tunes (this is a stock 2007 TBSS right?)

[WarWagon]

Ok caught the bucking getting bad and then forced 1st gear and powerbraked through the range again. Frame 102-103 shows an RPM drop. Frame 9-10 misfire was just after engine start in park.

2006 2wd TBSS. Opened up headlight air intake, stock filter and intake. Tune is from AZ Power and Sound now out of business. This had the surge with the stock tune. Cleaning MAF and changing air filter did not help. I do have a Yank 3200 stall in the garage waiting for this issue to be solved 1st. Stock except for the tune at this time.

I did have them "bring it on" for the cooling fan for the 121 degree summers here. I think we could have left out more fan on the engine but called more for the AC?

Adding 'driving around' file for reference.

[BBA]

Have you checked for glowing plug wires at night in the dark?

I'm betting since you had a misfire code...it's a bad wire.

[WarWagon]

Frame 123 picks up a miss on cyl #7. (#4 missed last startup above.) Is the lean shown from the o2 sensors a result of this miss or did the computer go too lean?

I also see a bit of KR. 5.5 Deg in the first driving log and 2 deg in this log. Is this enough to be concerned about?

I'll check the wires. I had a MAF fail on an 85 Olds and it lit the wires up like an xmas tree going so lean.

[bwl101]

Any KR over 5 is something to be concerned about, although 5 seems to be common on tip in on many tunes...even stock tunes...

[Russ K]

The LTFT's are positive because the low Maf table is lower than stock, and the high Maf table is lower than stock till 8000 Hz. I would start by returning the low Maf table to stock, and returning the high Maf table to stock up to 7800 Hz. Then scan the truck & copy the LTFT histogram & paste special to the matching areas of the Maf tables to get the LTFT's closer to 0.

Also set up your LTFT map & rpm boundary tables as shown. You still have a PE delay, so make the changes as shown.

You can use my .cfg file, as the histograms are all set up for the Gen IV engines.

Russ Kemp

[WarWagon]

The changes are made except for the scan and LTFT part. This I am working on. I see my tune wasn't that good due to this not matching:
"make sure the last cell of the low matches the first cell of the high table."

However it runs better and no surging today.

[Russ K]

Sounds great, it will run perfect once you get the LTFT's closer to 0. For some reason, GM set the low, mid & high range rpm LTFT boundaries at 8192 rpm. So instead of having 16 fuel trim cells, only 4 were in use.

This is a stock 2006 Trail Blazer SS tune for you to compare. Did you make the changes to your Maf tables?

Russ Kemp

[WarWagon]

Sounds great, it will run perfect once you get the LTFT's closer to 0. For some reason, GM set the low, mid & high range rpm LTFT boundaries at 8192 rpm. So instead of having 16 fuel trim cells, only 4 were in use.

This is a stock 2006 Trail Blazer SS tune for you to compare. Did you make the changes to your Maf tables?

Russ Kemp

Thanks for the file, I was using one that was put in the tune repository.
I have made the suggested changes and am now tweeking the MAF from the LTFT scans.

My last at temp run still shows #7 and #8 missing a little. How much misfire do these engines normally have if any?

[Russ K]

The odd misfire is ok, but if the misfire counts are steadily increasing, that is a real misfire. Is the idle noticeably rough?

Russ Kemp

[WarWagon]

The odd misfire is ok, but if the misfire counts are steadily increasing, that is a real misfire. Is the idle noticeably rough?

Russ Kemp

Well now that you point it out... The timing is all over the place at idle. It isn't missing but you can feel a shake. I did not think these had a smooth idle, but it looks like if the timing held at one point it would.
I have set the MAF back to stock again and cleared the LTFT. (Missed that step after going back to stock.)

Water spray test gave me a misfire only once so I think it was random and no vac leaks.

Log of timing from negative numbers to 20 Degrees at idle posted. This was from the MAF corrected stock readings. So changed from stock on old LTFT.

[Russ K]

That random misfire is normal. Your fuel trims look very good now! The timing will jump around at idle to control the idle speed. Post your latest tune, and I'll take a look at the min airflow table and the idle over & under speed spark tables.

Russ Kemp

[WarWagon]

I am going to redo the fuel trim part of the tune. It just did not run as good as stock MAF settings and the numbers look to be going down by -1 in readings. So I think the old LTFT were affecting and over shot it after the return to stock.

So MAF settings above 7800 were changed per the trims. The rest are returned to stock and I will be adjusting these this week.

While you are in there what do you think of the A/C and ECT EV fan % setting revision?

My goal is good AC at idle and not letting things get out of hand when towing.
(Stock belt fan) Stock is not enough fan for the A/C when it is over 110 degrees here and the first tune I put here was too aggressive on ECT I think. It did keep the AC cool at idle though. I am not used to the newer AC systems and their running pressures. (1995 134A sure, but not the new variable compressor pressures.)

[WarWagon]

Latest adjustment.
Idle all over the place with timing.

Two back to back scans show different corrections for this tune? 3 Hours difference and 49 degrees first run 39 degrees second run .

How should I add corrections if any?

[Russ K]

The idle spark is normal. You must bring the engine to operating temp BEFORE scanning. I see that you changed the Maf table above 7800 Hz. I thought I said to just change the maf up to 7800 Hz. Return the 8000 Hz & up cells back to the AZ Power tune, as this is the wot part of the Maf table. I assume that the wot AFR was checked with a wideband? You could use paste special multiply by % - half for the LTFT's as they are very close. Or you could set up a custom pid to use STFT+LTFT as I did for Misfortune;

http://www.hptuners.com/forum/showthread.php?t=15691

As for the spark knock, make the changes as shown. i multiplied the attack table by .5, and the decay table by 4. This will reduce the false knock, as you have way less than stock spark timing. Are you using at least 91 octane fuel?

The Fan Desired vs. ECT should work better. As for the Fan Desired vs. AC Pressure, you need to log the AC pressure to see what changes are needed. But when the ECT climbs, the EV fan will be 100% anyway.

Russ Kemp

[WarWagon]

The idle spark is normal. You must bring the engine to operating temp BEFORE scanning. I see that you changed the Maf table above 7800 Hz. I thought I said to just change the maf up to 7800 Hz. Return the 8000 Hz & up cells back to the AZ Power tune, as this is the wot part of the Maf table. I assume that the wot AFR was checked with a wideband? You could use paste special multiply by % - half for the LTFT's as they are very close. Or you could set up a custom pid to use STFT+LTFT as I did for Misfortune;

http://www.hptuners.com/forum/showthread.php?t=15691

As for the spark knock, make the changes as shown. i multiplied the attack table by .5, and the decay table by 4. This will reduce the false knock, as you have way less than stock spark timing. Are you using at least 91 octane fuel?

The Fan Desired vs. ECT should work better. As for the Fan Desired vs. AC Pressure, you need to log the AC pressure to see what changes are needed. But when the ECT climbs, the EV fan will be 100% anyway.

Russ Kemp

At temp.... Ok I was looking for the cold surge and overlooked that. This last tune file still has the surge in it. (So does stock tune.)

91 Octane fuel always.

Dyno Tuned in 100 degree weather. Not sure if wideband was used. I assume that high outside air temps is why less spark is used? Should I go back to a stock spark table? (I have not messed with the spark myself.)

I am comparing the idle to Misfortune and his is dead steady for the timing at idle. Notice I idle at temperature at the end of the first log and timing is still all over the place. Maybe lower than stock timing is doing this at idle to me?

Thanks for the fan idea. I will log AC this summer and see if it makes any difference over ECT's.

[WarWagon]

After looking at the sustained 4.5 Sec of knock at freeway speed in this log there is no doubt that the spark plugs have a new part number - reserved for Diesel Glow Plugs!
I thought I bought a van on the passenger side changing to NGK TR6, one heat range colder. What a pain.

The factory plugs were bone white with a slight pink tint to them. No deposits what-so-ever. So I say they were running too hot.

The Knock file is before the changes made in this tune as Russ suggested. (Tune last one posted here.)

The after file, CH, is both plugs and tune change.

Current tune posted.