sncboom2k

Hi everyone - I'm turning to the guru's for assistance. Finding a qualified LT1 mechanic now days in my area is virtually impossible.

The issue: When car is warm (closed loop) it runs great - plenty of power - engine was just rebuilt and we added a TPIS ZZ9 cam. However when warm, pull up to a stop light (car in gear - automatic trans) - the engine gets choppy, starts to lope then eventually dies. If I put it in neutral - it stays running just fine.

The engine is a standard rebuild with no porting or changes to the throttle body. Added were Isky springs, Comp Cam rocker rollers (1.5 factory size) and the ZZ9 cam.
Fuel pressure is at 36, intake manifold vacuum is at 13 (which TPIS tells me is normal for this cam but it seems a bit low to me as I read 17-19 for a stock LT1)
Oxygen sensors are new, cats are new, EGR is good - smoke test showed no signs of a vacuum leak.
Opti is an Accel with 12K miles on it (which was working great prior to the rebuild) - I may need to test it with an o-scope.
Plugs and wires are new (AC delco factory plug and Summit Racing wire set). Just replaced the coil - no change.

We read everything with my snap on computer - LT Trim shows a small differential between bank 1 and 2. Fuel banks show 3.8 and 4. Close but shouldn't these be identical?

If anyone can provide some guidance, I'd appreciate it. If anyone knows the settings for an o-scope to test the Opti - I'd appreciate if you could share those. I know it's AC and I'd be looking for a square wave. (Watched the videos).

Thanks all!

Injuneer

If Bank 1 average LTFT is +3.8%, and Bank 2 average LTFT is +4.0%, they are virtually identical. Small differences are easily explained with minor difference in injector flow rates, minor reversion due to the more aggressive cam, minor differences in the O2 sensors, etc.

Question is, are those the average values for the LTFT's (all cells), or for one specific fuel trim cell (there are 18 fuel trim cells, all but three arranged in a grid defined by RPM and MAP)? You can learn more by pulling up the LTFT's for as many Cells as possible. Since your problem appears to be only at idle, Cell 16 (idle) is critical. But it needs to be compared to other low load/low RPM cells to try and identify the cause.

No codes? Fuel pressure within spec? Trans seem OK, stock converter? Have you used the scanner to watch the idle air control (IAC) counts to see how it responds to the faltering idle RPM. Can we assume TPIS supplied the tune? If not, who tuned it?

Stock LT1 (at sea level) will pull 20 "Hg vacuum. No idea about the ZZ9, but does sound low. Valve adjustment, misfires or incorrect ignition timing could account for it being low, if that's the case.

The Opti puts out two square wave pulse signals alternating between 0 volts and 5 volts. Low resolution signal has four variable width pulses per crank revolution (variable widths define which cylinder is approaching TDC), used to time ignition and injection. High resolution has 180 equal width (1° off/1° on) pulses used only to define the exact engine RPM in between low resolution pulses. The Accel is not one of the better choices. Unfortunately, there are fewer decent units available as time passes.

GaryDoug

Here are some o-scope screens showing the low resolution pulses (top trace) and the hi res pulses (bottom trace). The signals are the same in each shot. In each one, the time-base (shown at the bottom of the screen in white characters in time per division on the screen) has been changed to show the selected waveform (low res or hi res) in the best way. This was done with the outputs pulled up to 12 volts so the voltage is higher than the normal 5 volts. These were taken using an artificial way to rotate the shaft outside an engine, so they may be in the opposite direction from normal. Also shown is a photo of the timing wheel with explanations of each signals function relating to cylinder number. For the low res signal, there are 8 pulses per cam revolution, 4 same-length narrow ones and 4 variable-length wider ones.

sncboom2k

Thank you both very much for the information - My O-scope looks exactly like the one in the pics (Rigol DS-1052E) - The car will be back to me on Monday and I will test the Opti 1st and foremost.

To answer some questions:
No codes? Fuel pressure within spec? Trans seem OK, stock converter? Have you used the scanner to watch the idle air control (IAC) counts to see how it responds to the faltering idle RPM. Can we assume TPIS supplied the tune? If not, who tuned it?

No codes - initially had a P0300 - but only once. Fuel pressure is at 36 - within specs. Transmission seems ok - yes stock converter. I do not remember watching the IAC while it was stumbling but will do so. There is no tune in the ECM. Factory only at this point.

The 3.8 and 4 I'm referring to were the injector pulse width. I am attaching the initial readings a mechanic found (they matched my readings) - he then found several grounding issues and corrected them. I will post the latest info once I get the car back on Monday.

Thanks again guys - can't wait to get the car back and work on it myself again.

terminal_crazy

Has the pcm been reprogrammed?

Either iac valve openings altered.
Idle speed increased slightly
Closed and open throttle timings all help motor recover on a fast decel and stop.

Mitch

Injuneer

sncboom2k - for some reason the system is putting your posts into the "moderation" queue. When that happens, the post won’t show up until a moderator approves it. Sorry for the inconvenience, so please be patient. Not sure what in the posts is setting the system off.

You really need to get it tuned when you change the cam. Specially with TPIS's huge 14° split on intake vs. exhaust, and the 112° LSA. Their listing indicates it will produce "a slightly choppy idle, making it sound like you have a serious powerplant under the hood". And finding a good LT1 tuner is getting harder every day.

I'll download the file later and take a look at it.

sncboom2k

Thanks all - and no - I do not have any tuning at this point. Factory ECM. I'm going to call TPIS again tomorrow and see if they confirm this 13" HG vacuum is in fact normal for that cam.

The last mechanic altered the TPS to idle the car at 850rpm's - which didn't help the stalling at idle in drive issue. I have been told of a good tuning shop nearby (about 30 miles away). I'm going on a recommendation only. I'll call them and see what they know about my LT1.

If the opti tests good - it may just need to be tuned. It idles like crap when warm - but has a ton of power when you stomp on it. TPIS told me I'd only get about 70% of the benefit without a tune. I planned to get it tuned but felt it should run correctly without one before doing so. Am I incorrect in that assumption? This is my 1st rodeo as far as rebuilding the engine is concerned.

I'm ignorant in the ways of stall converters but a mechanic mentioned I should get an 1800 stall converter installed for daily driving. Anyone agree or disagree? I don't want to split the trans from the engine if I don't have to.

As always - thank you everyone - I appreciate your help in getting me in the right direction to being able to enjoy this rebuild.

sncboom2k

Looking like the tune may be the requirement regardless of what TPIS is telling folks now days. Just found this in another thread about the ZZ9. (I'm still going to test everything - then try to find a tune after (if any) repairs are needed.)

Posted in 2003 -
Im running it in my car and am getting 31mpg with stock 3.07 gears and an auto. I would suggest programming with an auto unless you like riding the brake at stoplights trying to keep it from dying. I am running 12.7s with a lot of bolt ons, factory heads and bottom end, on Nitto D/Rs. I am pretty happy with it I would say, but am switching up to the XE 224/230 grind with AFR 190 heads.....anyone need a zz-9 with 1500 miles?

sncboom2k

And the search for a tuner is becoming more troublesome. I called the recommended tuning shop today and they told me no they don't do LT1 motors and didn't know of anyone who did. Damn. Anyone know any in the Los Angeles, CA area? If not - who can recommend a good mail order tune? May be my only option if they all keep telling me no.

Thanks all.

Injuneer

Guess I shouldn't have ASSumed that your "3.8 and 4" were fuel trims. Unfortunately, you have a serious problem with "split LTFT's". Many possible causes, but one of them can be lack of a tune with a larger cam. Since the LTFT's are "multipliers" applied to the fuel calculation, the injector pulse width mismatch is caused by the LTFT's. Your pulse width variations are larger than you quoted.

Looking at the four data sheets, which unfortunately are not showing all the same sensors and parameters, making comparisons or trending difficult:

1:

Your mechanic highlighted the issues - coolant temp is on the high side, but not beyond limits. Normal operating temp is designed to be about 210-degF. The fans will not even turn on at low speed until 226-degF, high speed at 235-degF

He highlighted 127 STFT, noting that 128 is "perfect". This is a single frame snapshot. The STFT's are toggled up and down up to 9 times per second, between slightly rich and slightly lean to allow the cats to work. The fact that it reads 127 in a single "freeze frame" is meaningless.

He highlighted the big difference between the Bank 1 and Bank 2 LTFT's. Bank 1 is subtracting 10% from the fuel the PCM would normally supply. Bank 2 is adding 8% to the fuel the PCM would normally supply. Anything beyond +/- 5% indicates that the cause needs to be investigated. Assuming the 36 PSI fuel pressure was at idle with the vacuum compensation line connected to the fuel pressure regulator, that is correct. Without the vacuum line connected, you should see 43.5 PSI (GM accepts 41-47 PSI as within range).

This is where the pulse widths would have been helpful, because it would have been easy to show that the difference in LTFT's is (roughly) mirrored in the difference in pulse widths.

As mentioned, the difference between left and right could be caused by lack of tune. Aftermarket throttle bodies can also cause this problem, but you indicate "no changes" to the throttle body.

The MAF reading at 7.94 grams/second is normal.

Quick lesson in interpreting the data - intake manifold vacuum = barometer minus MAP. In this case, BARO 30.4 "Hg - MAP 14.1 "Hg = 16.3" intake manifold vacuum. Not as bad as you indicated (will address that in comments to sheet #4).

Mechanic suggested possible O2 sensor problem - swap the two O2 sensors side-to-side to see if the mismatch follows the sensors (sensor problem), or stays the way it is (not a sensor problem, but possibly a wiring problem).

2:

With 0% throttle position, would assume this is with the engine idling in neutral. Idle RPM is elevated a bit at 882 RPM. But this may is due to the low (127-degF) coolant temp. Stock programming idle, engine fully warmed up, with auto trans is 550 RPM in gear and 650 RPM in neutral. But between 111 - 132-degF the PCM elevates the idle over a range of 800-750 RPM in gear and over a range 900-850 RPM in neutral.

Misfires cause the PCM to elevate the LTFT's to add more fuel. The O2 sensor "sees" the oxygen that should have been consumed by the fuel, but wasn't. Cylinder 6 would explain the + LTFT correction on Bank 2. But Cyl 7 on the other side also seems to have a high misfire rate. Contradicts Bank 1 subtracting fuel. But this record does not cover LTFT's or fuel trim cell number. But the Bank 1 3.7 mSec pulse with, vs Bank 2 4.3 mSec pulse width equates to a 16% difference, which is similar to the difference in LTFT's seen in other sheets.

This sheet has MAP at 10.6" Hg, which would indicate a 20 "Hg vacuum with a standard Barometer, but Barometer data is not shown.



I'm going to post this now and cover the other 2 data sheets in the next post.

sncboom2k

Thank you Injuneer - I greatly appreciate this analysis. The pdf file you're looking at is prior to the fixes he performed on missing and broken grounds. So the #'s of 3.8 and 4 that I provided you were given to me verbally by him after that. The car is on its way back so I can pull fresh sensor data from it this evening and post those.

That is correct - the intake and throttle body are oem. As are the injectors and fuel pressure regulator. I will pull the vac off the FPR and see if it jumps up. I'll take pics of the gauges I have for manifold vacuum and fuel pressure and post those as well.

Sounds as though I may need to check my Cylinder 6 and 7 plugs / wires? (All new but we know that means literally nothing with the quality of parts now days.)

I spoke to Clay at TPIS again today - he assured me that the 13"HG vacuum was normal for the ZZ9 cam - he said 14 but 13 was ok as well.

Thanks again!

Injuneer

3:

RPM right where it should be

TPS volts within spec

-3% LTFT on Bank 1 vs. +13% on Bank 2 is 16% split - excessive

In this record., Barometer 27.7 "Hg - MAP 14.1 "Hg = 13.6 "Hg vacuum

Coolant better at 194-degF

Intake air is hot at 105-degF, but I don't know what ambient is where you live

This one confirms the PCM is operating in Cell 16, which is the "idle" cell (throttle position % = 0, vehicle MPH = 0 defines "idle").

The absence of knock retard is good, and the ignition advance at 21-deg is typical for idle.


4:

Idle = 666 RPM.... that's ominous

Again, some misfires indicated.

Coolant OK Kat 206-degF

Cylinders 1, 6, and 7 showing high cumulative misfires. May need to pull the cap off the Opti and look at condition of rotor tip and cylinder contacts.

I see someone circled Map of 13.2 "Hg. As noted in earlier discussion, that's not the vacuum. Since barometer is not recorded, sort of meaningless.

No LTFT's in this sheet, but injector pulse widths are indicating a 23.5% difference between right and left banks.



Not sure we learned a lot here, but at least we have quantified the form the problem is showing up in. Observation of the IAC counts, and how they respond to the loss of RPM at hot idle will still be useful.

I went through a similar problem recently with an individual who had done the 24X conversion, running his LT1 with LS1 PCM, no Opti, and 8 coils. We went around in circles, until he pulled the plugs at my suggestion, and turned out one of his cylinders had a bad coil, plug definitely had never been fired. Unfortunately, it appeared someone had turned of the misfire detection and possible ignition coil control system checks (per cylinder), or maybe that's a byproduct of the 24X conversion.

sncboom2k

Thank you Injuneer - very useful information. I am appreciating the education.

Car is home now - I have checked a few things and will dig in more tonight.
Fuel Pressure at key on car off - 41
Car running at idle - warm - 38
Car running at idle - warm - vacuum removed from the pressure regulator - 46
Intake manifold vacuum at idle - warm - using gauge - 14"HG (Idle is close to 850 now)

I'll hook up the computer and see if I can get the other readings for a fresh set tonight. Then I'm going to figure out where to connect my o-scope to look at the Opti Spark signals.

sncboom2k

OK here is dang near everything my Snap On Solus would provide. Of course some switching data for the O2 sensors is missing as I couldn't record it. It won't let me save the screen shots to the computer since i don't have a working battery. So I typed them all out. Here goes.

RPM (840-860)
KS Counter 23325
Coolant Temp (222)
AF Ratio 14.7:1
MAP"hg (12.9-13.2)
BARO"hg (30.4)
BARO V (4.9)
IGN Volage (13.3)
Low Oil Level (Yes) ???
Engine Load % (4)
FT Cell (16)
High Res Signal (On)
FT Learn (Yes)
Knock Retard % (0)
LT TRIM AVG B1 (124)
LT TRIM AVG B2 (129)
LT TRIM-1 (-10)
LT TRIM-2 (9)
MAF gm/sec (10.7)
MAP V (1.89)
Intake Air (96)
IAC Position (0)
IAC Memory (10)
Spark Advance (12)
RPM Fine Res (560ish)
RPM Lo Res (degree) (2.5-3.2)
Lo Res Sig ms (16.8-17.5)
Inj PW B1 - (3.8)
Inj PW B2 - (4.4-4.6)

Misfire History
1 - 395
2 - 66
3 - 263
4 - 466
5 - 4
6 - 2
7 - 16
8 - 0

RPM Low Res - 2.85 to 3.26 AVG
ST Trim AVG B1 - 125
ST Trim AVG B2 - 130
ST Trim-1 (-2)
ST Trim-2 (2)
TPS Norm (%) (0)
TPS V (.63)

Getting a P0300 code but no Service Engine Light
I cleared the code - shut off the car - started it again - and the code is back per the Snap On computer. But no SEL.

Going to dig into the Optispark with the o-scope next.

sncboom2k

Ok so I did some testing tonight on the ICM and the Opti signals.
1st - I removed and ohmed out the Opti connecting cable - all pins .5 ohms - it's good.

2nd - all voltages were correct at the ICM per shbox's website.

3rd - See pics of the signals from the PCM at the white wire on the ICM. It looked ok to me - guru's please let me know. (Last 2 attached pics taken with AUTO mode on.)

4th - See the diagram attached of the wiring I used to obtain the signals.

5th - I was able to get 1 of the signals (I believe the high resolution) directly from the opti. I was not successful in getting a good image of the low resolution?. Not sure what settings I need to be using on my o-scope. Voltages were not the same as listed above since I didn't pull anything up to 12 volts etc.

Attached are the images I got.