LT1 Based Engine Tech 1993-1997 LT1/LT4 Engine Related

LT1 miss fire

Old 03-16-2019, 03:04 PM
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LT1 miss fire

kinda long, will try to be brief.

the car:

1995 roadmaster, bought it for my son's first - it's too big/ugly/whatever -
now I drive it as a daily and abuse it sometimes at the dragstrip.

3.42s, yukon duragrip, 245/60-15s all around

fresh valve springs

home depot intake

speed daddy stainless headers [1 5/8 'clipster']

2.5" exhaust, h pipe

WBO2

EGR/ABS/AC/A.I.R deleted

fresh 4l60e with 10"converter [no dragstrip times on this converter yet]

tuned by me

Best ET to date: 14.66 @ 92.5 mph, 2.045 60'- 1800' DA, 4450# as driven.



the problem (been chasing this issue since late june):

every once in a while the car would miss fire/sputter.
usually [but not always] on rapid throttle transition.

I had a hard time getting it to occur/repeat.
happened to me maybe once every other week.

getting out of it [throttle] and back in seemed to help some.
hold it to the floor and it will continue to miss fire.

10 mins later the car is fine [wtf?]

Found coolant getting into #8 cylinder, replaced long block with a 40k mile one. Motor came with a Delco opti (bonus).

Still have a sputter at idle in closed loop only. Seems to want some heat in the motor before it will manifest.
wideband (installed on driver's side) will show a lean dip to 16.xx:1 when the engine is sputtering.

if I lightly touch the pedal when it is sputtering (idle), the car will come out of it.

it will sometimes continue to sputter on initial acceleration.


WOT pulls will sometimes sputter and WBO2 will read lean (~14.1ish)


what I have done so far:

replaced bad MAF

refreshed the Delco opti with the MSD C&R kit, new vac lines, etc.

new opti pigtail

new plugs/wires

verified fuel pressure [got lucky and had the car act up with the gauge
taped to the windshield] at WOT thru 1st gear at ~50 psi and not dropping.

41 psi key on engine off, 38 psi after 35 minutes time.

swapped coils, new icm and cts and all new pigtails/connectors

new tps

used a spacer to get the icm/coil off the head a bit

new denso O2 sensors

Vac gauge tells me 23" at hot idle in park.

not seeing anything odd in my data logs

cleaned IAC/throttle body

checked 1 3 5 7 wires with a timing light, all looked fine. no intermittent pulse on any while it sputters.

was able to shut down injectors 1 at a time to see if/how it affected the sputter behavior - I couldn't nail it down to any 1 cylinder...

IAC counts are hovering around 16 (throttle blades are closed), bank 1 blms were 135 in cell 16, makes me think vacuum leak (not that I can find one)

did notice some slow vac leakage on the evap solenoid to canister line (w/solenoid off) so I capped the fitting and drove it a bit, idle sputter remains.

cleaned up 4 of my spare injectors and swapped out the drivers side ones this morning, idle problem remains.

I have a spare pcm - might throw it in just to see...
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Old 03-17-2019, 02:46 PM
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Re: LT1 miss fire

Found coolant getting into #8 cylinder, replaced long block with a 40k mile one. Motor came with a Delco opti (bonus).
To clarify - did you have this misfire problem both before and after you replaced the long block? If so, did you reuse the fuel rail/injectors/fuel pressure regulator from the older engine?

Fuel pressure is on the high side, outside GM spec (41-47 PSI with no vacuum). The PCM is programmed for flow based on the design pressure of 43.5 PSI. Injectors are flowing 7.2% extra fuel, which would normally cause the LTFT's (BLM's) to drop below 128. Something is not right. What is the fuel pressure at idle, both with and without the vacuum compensation line attached to the fuel pressure regulator?
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Old 03-17-2019, 04:08 PM
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Re: LT1 miss fire

Originally Posted by Injuneer View Post
To clarify - did you have this misfire problem both before and after you replaced the long block?
yes. I was optimistic that the coolant getting into #8 was the source of my miss but that was not the case.

If so, did you reuse the fuel rail/injectors/fuel pressure regulator from the older engine?
yes - until yesterday when I swapped out 1,3,5, & 7 injectors as a test. My thought process may be hard to follow but I think the miss is a random no-pulse on the injectors. Swapping the injectors had no effect...

Fuel pressure is on the high side, outside GM spec (41-47 PSI with no vacuum). The PCM is programmed for flow based on the design pressure of 43.5 PSI. Injectors are flowing 7.2% extra fuel, which would normally cause the LTFT's (BLM's) to drop below 128. Something is not right. What is the fuel pressure at idle, both with and without the vacuum compensation line attached to the fuel pressure regulator?
was using a parts store loaner gauge and trying to read WOT while driving...
I can guarantee the needle was not dropping during sustained WOT with the motor missing.
Not so sure my old eyes can tell the difference between 48 and 50 psi at a glance or using peripheral vision (trying to watch the road & gauge at the same time). Also not 100% confident that the gauge is accurate. Been there before...
I do not recall the p.s.i. numbers at idle but IIRC they were within spec.
I did not check at idle with the vac line disconnected from the regulator - was more concerned how it acted while driving the car.

I swapped in my spare pcm earlier today (made no change but at least I can rule it out) and logged the car (eehack) on the drive home. Haven't scoured the log yet but any of the records after about #4000 where you can see the wideband dropping to 16.xx at idle the car will be missing.
Log file attached, I simply renamed the file extension to .zip to upload (valid file extensions and all that) - it will need to be renamed .eedata before trying to view it.

Thanks for looking.

Edit added: All 16 injectors ohmed to 12.5 at ambient temp (~50 deg. F)
Attached Files
File Type: zip
spare pcm.zip (1.22 MB, 1 views)

Last edited by Rhansen; 03-17-2019 at 04:18 PM.
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Old 03-17-2019, 07:40 PM
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Re: LT1 miss fire

A decent fuel pressure gauge is a good investment. Disconnecting the vacuum line at idle is relevant to “driving the car”, because it replicates what happens at WOT when vacuum all but disappears.

As much as I'd like to help reviewing the data log, every time I look at an eehack log they are extremely cumbersome to manipulate into the Excel format that I've developed to efficiently review the thousands of lines of data. Have you ever found an injector circuit trouble code in one of your logs? Is the fuel pressure regulator stock or aftermarket?

An intermittent problem with an injector, injector circuit, or ignition seems logical.

Is there a single wideband? On one bank or both banks merged?

Are you planning to check the bank 2 wires and replace the right ban injectors?
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Old 03-17-2019, 08:33 PM
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Re: LT1 miss fire

For what it is worth, my free scan app can do this car.

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Old 03-18-2019, 07:32 AM
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Re: LT1 miss fire



I can install and use scan9495, just haven't done (needed) it yet. Add it to my list...

Fuel pressure rgulator is stock and not leaking.

I loaned out my fuel pressure gauge years ago and it was never returned. Haven't had enough use for one to buy another yet (until now apparently).

Again, I did do WOT testing with the pressure gauge, but only while driving the car. Simple enough (and safer) to do it again with the vac line off and the car parked.

My (single) WBO2 is installed just after the drivers side O2, before the H pipe.

I have been focused on bank one because that is the side where I have the best info and ease of access. Ever tried getting a timing light on the pass side wires with the car running? High probability of 'searing the meat' on my headers. Since I know that there is a problem on bank1 I am working that side first.

AFAIK the injector circuit malfunction codes in the 8051 pcm are not utilised. I can activate the flags but am told this is untested. Simple enough to reflash the pcm with those flags checked. (see attached screenshot).

Will not be replaceing/swapping any more injectors until I can nail down this problem and/or get a set professionally serviced. My spare injectors have been sitting for 6+ years and were a bit sticky. I freed 4 of them with a heat gun and 50 psi of carb cleaner then techron put through them while pulsing manually with a battery. Not ideal but the best I can do with what I have on hand.

Thanks for the feedback, will reflash pcm with injector fault flags set and get a scan or two with scan9495. This may take me a few days, work is hectic atm.
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Old 03-18-2019, 07:58 AM
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Re: LT1 miss fire

Seems like my comments and questions aren't helping much. Seems you have already covered everything I've mentioned.
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Old 03-18-2019, 05:31 PM
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Re: LT1 miss fire

Just tried running a scan with GaryDoug's tool but it keeps crashing & will not connect. Will have to play with it more later...
Win Vista laptop, .NET framework 4.5.2, 16 pin cable from here: OBD Diagnostics

Seems like my comments and questions aren't helping much
you're helping plenty - no worries.
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Old 03-22-2019, 05:22 PM
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Re: LT1 miss fire

Managed to get a log with scan9495 on the drive home from work today.
Car was running extra craptastic too
Attached Files
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Old 03-23-2019, 09:54 AM
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Re: LT1 miss fire

I downloaded the file. I should be able to take a look at it later today or tomorrow.
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Old 03-24-2019, 01:25 PM
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Re: LT1 miss fire

Well, it's official, I am an idiot.
Used a stethoscope to listen to the injectors, of course they all were switching fine.
So that brings me back to a spark issue. Before the engine swap the miss would not show on my wideband - but it did afterwards...hmm
Got the car good and warm then crawled underneath last night and viola, the #5 plug boot was arcing to the header tube (couldn't see it from above).
Pulled the header, dimpled the tube, replaced the boot (plug was ok), doubled up on header gaskets, and no more miss fire.
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Old 03-24-2019, 02:43 PM
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Re: LT1 miss fire

Originally Posted by Rhansen View Post
Well, it's official, I am an idiot.
Used a stethoscope to listen to the injectors, of course they all were switching fine.
So that brings me back to a spark issue. Before the engine swap the miss would not show on my wideband - but it did afterwards...hmm
Got the car good and warm then crawled underneath last night and viola, the #5 plug boot was arcing to the header tube (couldn't see it from above).
Pulled the header, dimpled the tube, replaced the boot (plug was ok), doubled up on header gaskets, and no more miss fire.
Good job. Perseverance wins again.

But I thought you replaced all the plug wires a week ago?
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Old 03-24-2019, 03:32 PM
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Re: LT1 miss fire

did new wires when I swapped the motor (January)
checked them (from above) several times...
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Old 03-24-2019, 06:32 PM
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Re: LT1 miss fire

Interesting, because after scanning fairly quickly through the 11,000+ line file, there wasn’t anything jumping out at me. Interesting thing is that only Cell 16 shows Bank 1 LTFT higher (139) than Bank 2 (133). All the other Cells were close to balanced, or higher on Bank 2. I generally look for values outside the normal tolerance of +/- 5% (122 - 134), and only Cells 5 (121/122), and Cell 16 (139/133) are outside the limits. 1, 2, & 3 were right on, 6 & 7 are pulling fuel on Bank 1/adding fuel on Bank 2; 10 & 11 are adding fuel on both Banks.

In general, looks like a healthy engine. No codes, MAF, MAP, TPS, pulse widths, idle RPM vs. target RPM, system voltage, O2 readings in PE mode look good. There are at least two instances of knock retard (I think 4 max, I'm not on my desktop) in response to relatively moderate increases in throttle opening, and decays very rapidly. There are some instances where the O2 sensors respond slowly, hanging at ~900 mV for a full second before starting to switch rapidly again.... didn't see a cause for that. Went into closed loop with only 90 coolant temp, seems unusual.

It was a fairly cursory review, and seems like you found the problem. I can look further, but doesn’t seem necessary.

Appears EGR has been disabled. EVAP still active. Are you using the A/C pressure sensor input to log the wide-band? No indication of any A/C component being utilized or called for, evaporator temp constant, but the A/C pressure reading is changing constantly up to 400 PSI.
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Old 03-24-2019, 08:23 PM
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Re: LT1 miss fire

yes, WBO2 is on the a/c press. circuit.

deleted the egr rather than modify the headers (lazy me)

I know the CAT overtemp protect routine will dump fuel frequently & often, especially after running any length of time in PE. It should be disabled in my tune but will keep an eye on it. The other possibility that comes to mind is that the average operating rpm of the motor is somewhat higher with the 10" converter. IIRC enrichment is triggered by elevated rpm as well as throttle position.

I'll bet that the BLMs in cell 16 will straighten out with the missfire gone. Missing was most frequent at idle oddly enough. On the log you viewed, the car was missing hard at idle and moderate (>35-40%) throttle position (thus the "extra craptastic" comment). I don't think I've ever had an arcing plug wire that didn't get way worse with even a little bit of added throttle before this.

Thinking the KR is an artifact of the missfire - time (and logging) will tell. Historically, this car runs KR free except when heat soaked and I stab the throttle hard/fast (i.e. to pass) and must wait briefly for the downshift to happen. There is also some resonance from the headers/exhaust (~2500 rpm) that will occasionally generate a few knock counts but not pull any timing.

I plan to do some logging/tuning before hitting the dragstrip this season.
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