View Full Version : 95 LT1 Problem -- Possessed by Satan

04-23-2012, 11:54 AM
My dad and I have been trying to fix his car for a year now, on and off. It started when he was driving home from a car show on a really hot summer day. The car started missing and he had to keep the RPMs up to keep it running. It eventually flooded and stalled. Around that same time, my dad's car started backfiring/popping as he backed off the gas after accelerating.

At some point in time early in the troubleshooting, we discovered that if the MAF was unplugged that the car wouldn't run at all - that struck us odd because I know my 1994 Camaro, cammed with an open loop tune, will run without the MAF plugged in. His wouldn't even start with the MAF unplugged. So we thought something might have been awry with the PCM.

We ended up having our tuner reflash the PCM and drove the 30-something miles home. It popped the whole way home, but we were told one of the O2s looked laggy and we might have an exhaust leak.

We replaced the one O2, that didn't fix it. We couldn't find any signs of an exhaust leak; all of the header bolts are tight. We tried swapping MAFs and ICMs to no luck. We pulled a few of the easier-to-access spark plugs and they were nice toast colored, not showing signs of lean or rich combustion.

We started datalogging using DataMaster and TunerPro RT and noticed split BLMs pointing to an issue with the right bank. We couldn't find any true source for it, so again we started wondering if something was wrong with the PCM. I read Gregrob's tutorial on using TunerPro RT and WinFlash and was able to download my dad's tune to my computer and look at it in TunerPro - it looked all weird - like the injector flow rate being 730+ lbs per hour and a cylinder volume of 0. The fuel per cylinder table (can't remember the right name) had cylinder 7 at 0.07.

I found a stock tune online, turned off the EGR/EVAP/AIR/etc and uploaded it back to the PCM. Immediately the popping was gone but after running for about 30 minutes we were able to capture in the datalog the same incident a year ago when his car started acting up and stalled. We watched as the short term fuel trims climbed steadily up (lean condition) and the injector BPW climbed from 3 to 8, flooding the car and causing it to stall. It was at this time we finally succumbed that it was probably the optispark.

This past weekend we changed the optispark and the blaster coil (my dad didn't believe me when I told him it was important to get it out of the way and he broke the coil plug off with his ratchet). Fired the car up and it sounded good, no popping. Once the car got to operating temperature it went into closed loop but the short term fuel trims immediately started climbing (lean condition). This time, though, the injectors stayed steady at 3.5BPW. The car didn't flood, it reached 194 INTs and triggered lean o2 codes and kicked back to open loop, recovering before stalling.

We're completely stumped. Later tonight I can upload TunerPro datalogs (XDL format) for people to review.

What we've replaced or swapped:
Blaster Coil
Both heated O2 sensors

What we have ruled out:
MAP sensor - MAP readings in comparison to BAR look OK in the datalog
TPS sensor - looks OK in datalog

What we haven't tested/checked:
Spark plugs
Fuel Pump

Aside from plugs and fuel pump, does anyone have any other ideas of what we could check? My dad's headers have a lot of surface rust, is it possible that the rust ate through the headers to make enough pin-sized holes to cause such a lean condition on BOTH banks? We've examined the headers and don't see any holes.

04-23-2012, 04:57 PM
Have you checked the fuel pressure regulator?

04-23-2012, 04:58 PM
Also, I thought Satan's Camaro was a 72 lol.

04-23-2012, 06:42 PM
Have you checked the fuel pressure regulator?

Haven't yet, but I'll add it to the list for tomorrow. To be honest prior to swapping the optispark this past weekend we never would have thought it was a fuel related issue aside from too much of it killing the engine. We actually had to change the oil after we replaced the optispark because we could smell gas in the oil - sure enough when we drained the oil there was more than 6 quarts of fluid that came out.

Anyway - now with the current conditions it's definitely something for us to check. Tomorrow when we get a fuel pressure tester from a buddy I'll check the regulator as well.

04-23-2012, 06:55 PM
All you have to do for the fpr is to pull the vacuum line off it. It's a very thin line that goes from the passenger side of the intake to the fpr at the back. Pull it off and check for fuel coming out of it or a fuel smell. Either of those is an indicator of a bad fpr.

04-23-2012, 08:04 PM
Yeah I know - the car is at my dad's house so I'm at his mercy on whether or not he's willing to check it - it's his own car, but he's been kind of disgusted with all the problems. I already left him a message to call me since it's so easy to check, but I haven't heard back from him. I'd love for it to be that. At this point it seems like it has to be something simple that we've overlooked -- I was so happy yesterday when I saw in the datalog that the IAT was reading 85* when it was only 50* out. We rushed to the parts store to buy a new IAT thinking that HAD to be the problem only to plug in the new IAT and get the same damn reading - didn't realize the air being sucked into a car on jack stands in a garage could be that much warmer compared to ambient temps. :D

04-23-2012, 08:16 PM
check fuel pump.. mine was doing this awhile ago... had to have it floor board just to drive 55 and it was back fire like crazy once i took my foot off gas it would just die
replace your fuel pump :) also might have faulty injectors too might as well replace those as well

04-23-2012, 08:16 PM
Bummer - called my dad again and he pulled the FPR vacuum line out. Granted the car has been sitting for a day so he said it wasn't wet with gas, but my dad took it to my mom to smell it and she doesn't smell any gas at all.

Tomorrow's tasks are fuel pressure tester and also to take starter fluid and spray it around the engine bay to see if we can spot any leaks that could be sucking in unmetered air.

Paul Bell
04-23-2012, 08:26 PM
Power is sent directly to the fuel injectors. The PCM grounds the other side of the injector as it needs.

When I had my '94, I had an intermittent ground in the harness from one injector to the PCM. This will dump fuel.

There's a way to test the injectors with a GM Tech 2. You'll also need a fuel pressure gauge. Through the tech tool, you can cycle the fuel pump on. Watch the gauge, confirm the pressure is correct and holds. Then you can do a test on each injector, one at a time. After each one, cycle the pump back on. Each injector should reduce the pressure by about the same amount.

04-23-2012, 08:42 PM
Thank god - only 3500! There's no way we can do that. We'll be checking the fuel pressure tomorrow.

I also forgot to mention we had the fuel injectors themselves all flow tested and calibrated because we originally thought the popping/backfiring was a leaky injector.

Paul Bell
04-23-2012, 08:49 PM
OK cool, you had them tested.

I had included the link so you could see the tool. Most better shops have it, you just gotta get them to do all the tests.

That's actually a good price, they go for over four grand.

Good luck with the fix! Every LT1 car has a little Satin in it.

04-23-2012, 09:18 PM
For anyone with TunerPro that's interested in looking at the datalog:

Paul Bell
04-23-2012, 09:25 PM
Hmmm, I just caught this. Testing the injectors outside the car does not confirm the wiring harnesses.

04-23-2012, 09:37 PM
Yes I realize that. Tomorrow we'll have a fuel pressure tester at our disposal so we'll check that and my dad was wanting to replace the fuel filter regardless of the results of the test. We'll also have starter fluid to spray around to see if there's any leaks letting unmetered air in.

If neither of those tests yield any results, we'll find a shop with a tech2 that can help us. Someone on my forum mentioned that if you hold a screwdriver against the injector you can hear it "clicking" as it fires. I don't know if that'd really work since it'd be clicking so fast, but is that worth a shot in lieu of the tech2? Just trying to stick to tests we can do in our garage at this point since it's up on jackstands for easy access to the under carriage.

Paul Bell
04-23-2012, 10:13 PM
How are all the plugs for the opti including the one into the unit itself? Any weirdness, corrosion, burned pins, etc?

04-24-2012, 12:05 AM
A bad ECT in the waterpump will cause the car to run rich and backfire/be barely driveable if it is bad enough.

04-24-2012, 01:34 AM
Here's the guide for testing the ECT in an LT1.

04-24-2012, 07:52 AM
Thanks for all the responses guys - I really appreciate it! All the ideas help us narrow in on what it can or can't be. It can't be the ECT because we're watching the data log and the temperature signal the ECT is sending to the PCM shows up in the data logs - it's in line with the gauge cluster's temp gauge. Plus we replaced the waterpump unit as a whole when we replaced the optispark as a maintenance precaution.

The wires going into the opti looked good as far as I could tell - the wire going from the optispark to the blaster coil is brand new.

04-24-2012, 04:40 PM
Well we know the problem is when it's in closed loop. That means IAT, MAF, or O2s. I'd check the wiring from the IAT to the PCM, judging on the computer reading 83* when it's 50* outside.

Also, couldn't you just spray starter fluid on the headers to see if they're sucking in air like w/ other exhaust leaks?

James Montigny
04-24-2012, 04:54 PM
Every LT1 car has a little Satan in it.
Yes, we call it the Optispark :jest:

James Montigny
04-24-2012, 05:13 PM
But seriously, check your grounds.
Take them off, one by one and clean them with a wire brush or sandpaper.
Do the same to the surface they mount against and re-install them.
Bad grounds have caused more issues around here than I can count and often leave people spending
themselves into a level of pissed off that you cannot imagine when they find the issue.

04-24-2012, 08:53 PM
Well - that was embarrassing. Apparently the vacuum line for the EGR (which has been deleted) was completely exposed. We found the rubber plug we had used and there was a crack running all the way from tip to head. It must have been allowing air in prior to the optispark swap but it was a small enough of a leak that the PCM could compensate.

When we did the optispark swap, I replaced the optispark harness and I must have knocked the rubber plug completely off the vacuum port and never noticed - the plug was sitting in the valley between the intake manifold and the valve cover.

Spraying carb cleaner around the engine bay exposed the issue real quick - as soon as I sprayed by the throttle body it became quite apparent that's where the issue was and then I realized what my dad and I both thought was a loud alternator whine was the damn vacuum port sucking air.

Lmao -- needless to say, damn embarrassed that we didn't catch that sooner.

Since the issue started last year - these are all the items that we found which were contributing to the problem:

1. Corrupt tune telling the PCM that the injectors were flowing 700+ lbs/hr.
2. One fuel injector was out of calibration by 15%
3. Bad o2 sensor
4. Bad optispark
5. Vacuum leak at the EGR port in the TB

Pretty nuts that we had all 5 of those gremlins that we were fighting all at once... no wonder we were so stumped that every time we thought we found the fix and there was still something wrong.

04-24-2012, 10:26 PM
*pats you on the back* The LT1 is the devil motor. Makes monkeys out of us all. So, how does she run now?

04-24-2012, 10:50 PM
your "corrupt" tune was probably caused by the wrong definition file. there is an EE and an EEb (seen mostly with 94s). if you see those jacked up numbers then switch to the other definition file.

04-25-2012, 10:33 AM
Thanks, Stevo - you're correct. I switched to $EEB and it shows correctly. Since the PCM is a 1995 PCM - shouldn't the tune be using the $EE definition?

04-25-2012, 04:10 PM
maybe it is a really really really early 95. i only know 1 person with an EEB (this was out of 2 95's and 5 94's) and it is definitely a 94.

04-25-2012, 06:26 PM
The weird part is that since we discovered the "corrupt tune" we have replaced it with two other tunes. I've only used tunes which were done using the $EE definition and they run perfectly fine -- could the $EEb be part of the reason why the "corrupt tune" didn't run good? So not so much that the tune was corrupt, but that the tune was done with the wrong revision?

04-26-2012, 04:53 PM
that i could not answer. both my 94 and 95 are EE 6 speed cars and the EEB is from an automatic so i never bothered to experiment on it. i dont know if the "eeb tune" is specific to the operating system in the PCM, the effects of loading the opposite over top of it or even how to externally identify one without attempting to read it. for all i know it was just a changeup to deter anyone not affiliated with GM from screwing with it.

02-13-2013, 08:22 AM
another way to check injectors. Pop them completely out and get a ohmeter. Jump the connections on the top and read them they should all be right around the same. If you have one that reads almost nothing theres your bad injector. Not sure exactly what our LT1'S readings are. But i figured out a mazda that had been giving us the same kinda problem for months with this test. Good Luck. also my lt1 bird is currently throwing a fit with backfireing and not running rite so let me know how you fare.

Paul Bell
02-13-2013, 07:05 PM
Hey ugod, you should check the date of the last post-in this case it was ten months ago. They probably have it now.

I've been guilty of not date checking also...

02-16-2013, 05:26 PM
I have a question fellas. 1995 z28 lt1 6speed with a comp cam. 290 duration .51 lift. Can the ecu or pcm be programmed to the cam? Noone in this f***ing town knows.

F0x Slaughter
02-16-2013, 05:35 PM
Can a PCM be tuned to a specific cam? No.
Can you tune a PCM for a cam? Yes.

You don't just go in the PCM and say "hey PCM I have this cam" and it sets it up.

02-16-2013, 05:43 PM
Can a PCM be tuned to a specific cam? No.
Can you tune a PCM for a cam? Yes.

You don't just go in the PCM and say "hey PCM I have this cam" and it sets it up.

ok i was told by everyone that the 95 pcm cant b tuned but the 96 and later can. Now i was also told that since the cams rpm rating is 1500-5500 the car really wont bring out the hp til it reaches 1500 rpm. but how can i tune it?

Paul Bell
02-16-2013, 06:02 PM
Incorrect. All PCM's can be tuned. In the world of fourth gen F-bodies, the 1993 was the odd guy out. 1994 and some 1995 cars were OBDI. Starting in 1995 and then out, they were OBDII.

Some '95 cars have what is unofficially considered OBD one and a half.

F0x Slaughter
02-16-2013, 06:07 PM
My car has OBD1.5 its OBD1 with an OBD2 connector.

93 has to have chips burned.
94 - 95 use TunerCats
96+ HPTuners.

When an item says 1500 - 5500 rpm it means its going to make its best power between those rpms.
Under 1500 its not going to be making max power.
Spinning it over 5500 its going to drop off.
When you tune you are setting your spark timing and your injector pulse.

Spark 'generally' you are trying to run the highest spark advance you can without seeing knock. This means the mixture is igniting soon as it can thus burning all the fuel/air making the most power.

Injector pulse is just what you are using to control the amount of fuel for a given amount of air. So say your Wideband is reading lean because you put a new CAI on. You need to make your injector stay open longer in order to spray more fuel to richen the mixture up.

02-17-2013, 09:49 AM
Hey ugod, you should check the date of the last post-in this case it was ten months ago. They probably have it now.

I've been guilty of not date checking also...

Worse yet - we sold the car because my dad was so fed up with trying to fix it. We never did get it fixed, never figured out what caused the popping/backfiring.

ok i was told by everyone that the 95 pcm cant b tuned but the 96 and later can. Now i was also told that since the cams rpm rating is 1500-5500 the car really wont bring out the hp til it reaches 1500 rpm. but how can i tune it?

What they probably meant was that they only have OBD2 tuning software at the shop. Even here in Detroit there's not too many tuners who tune OBD1 LT1s, most only do OBD2.

02-17-2013, 11:07 AM
So it wasn't any of the 5 things you mentioned above?

02-17-2013, 05:09 PM
ok thanks fox. ill go to tuner cats and c what i can do

02-17-2013, 05:36 PM
We replaced/swapped the IAT sensor, optispark, MAF, throttle body, o2 sensors, injectors, ICM, coil, spark plugs, spark wires, MAP... Probably more. Tried different stock tunes found online, etc. In the end we never could figure out what the problem was and we ultimately gave up. Every time we changed something it *seemed* to get a little better but then it'd start backfiring again. My dad was so disheartened by the whole thing that he didn't care to work on it anymore or drive it at all, he only put about 1000 miles on the car (what WAS a semi-daily driver except in snow) so he ended up selling it. I know the guy who bought it and hopefully he'll figure it out.