I’m out of ideas now. Random misfire, hard start.

[Road Trip]

Still trying to get some insight on this, when I feel like I’ve narrowed it down, I’m just back to square one. Sorry for the long post.

Did a lt4 hot cam, not sure if that will cause the misfire, tuned ecm from black bear for it.

97 5.7l , random misfire, shakes every few seconds or so at idle. First start of the day it fires right up, will crank and crank and crank unless I crack the throttle just a little bit any start after that. Even 3 seconds of running on the first start of the day.

Now this is where I’m stuck at, when i unplug the MAP sensor truck fires right up no problem. I sprayed everything I could up top with brakleen for vacuum leaks, no difference in idle.
It’s showing misfires on every cylinder except 8. 2 the most, plug looks fine.

I do have a mail order tune from black bear performance. He’s very knowledgeable but hard to get a hold of. I have narrowed something down though, he just happen to get back to me and say that my tune target timing at idle should be 26-28*. On my scanner I’m looking at 36* timing, cam retard at 0. Now I do not know how to change the timing without changing the retard.

Hello again Dlipp_,

Let's see, since you're driving a '97 I need to switch to the Vortec perspective, offer a few bits of info, and ask
what visibility you have regarding the live data from your VCM, in that order.

First things first. For some reason I initially got the impression that your engine bay was under TBI control. My bad
for not double checking, but by the same token the ability to access live data (and eventually tuning this yourself)
will be a lot easier in Vortec OBD2 land versus trying to set yourself up for the same capability in the OBD1-era TBI setup.

OK, normally with hard starting the first variable we need to validate is the correct fuel pressure. According to your
parts list, both the fuel pump & fuel filter have been changed out, and I'm assuming that the ground connection
were brought back to new condition while the pump was replaced. (G404?)

But what I found interesting is that by disconnecting the MAP sensor your starting issues disappear? According to
the '97 Factory Service Manual, the MAP and the TPS sensors are used to calculate fuel delivery. And *if* the VCM
detects a malfunction in the MAP circuit, it will substitute a calculated value for the MAP value so acceptable
performance is maintained:

(taken from the '97 Factory Service Manual)

You must be registered for see images attach

From reading your original post, I know that the MAP sensor has already been replaced, so
here is where Mr. Schurkey would want to know what MAP readings your VCM is reporting.
(Problem in the wiring harness, bad out of the box new sensor of dubious parentage, etc)

This would be the next logical step, but for the moment let's assume that you don't currently
have access to live data like that. Instead, let's focus on the starting behavior for right now.

If I interpreted what you wrote correctly, you are saying that the first start is always good,
but subsequent starts are long cranking affairs unless you slightly depress the gas pedal?
(With the MAP sensor connected.)

But if you disconnect the MAP sensor, the starting is always good?

Coming at this from a slightly different direction, if the timing at idle is currently at 36° at idle with
the MAP sensor connected, does it change back to the tuner's spec of 26-28° if you were to
disconnect the MAP sensor?

Q: One thing you haven't mentioned is whether or not you have a SES/CEL light illuminated at
any point? Are there any DTC codes being stored, whether or not the SES light is on?

Obviously at this point I have more questions than answers, but hopefully we can sort out what's
going on and start making some troubleshooting progress.

****

By the way, when it comes to ignition timing in Vortec-land, please keep in mind that ALL your initial
(base) timing is set by Mr. CKP, and has nothing to do with twisting the distributor housing position. (See
attachments #1 & #2 for details.)

As for the 'Cam Retard' value, if it was up to me I would rename it 'Rotor Tip to Spark Plug Wire Terminal Alignment' value.
In English, changing the Cam Retard reading will not alter when the spark plug will fire
inside the combustion chamber. On the other hand, if you get the Cam Retard far enough out of spec,
you may end up with the wrong spark plug terminal closer to the rotor tip than the correct one...and sending
sparks at the right time yet down the wrong spark wire to the wrong cylinder is obviously a sub-optimal tuning strategy.

So, please leave the Cam Retard set to 0°, and let's instead figure out why your timing isn't where
your tuner expects it to be. And no better place to start than to figure out why your engine is
starting better with the MAP disconnected.

If you reply back with the status of your DTC codes, your SES light, spark timing difference (if any) at
idle with your MAP disconnected, and what you are currently using to read data from your computer,
then maybe we can start to tighten up the problem definition.

One last thing. I'm assuming that you are running a black box VCM with a known-good custom tune from
Black Bear, and once we get all the sensors verified good that the truck will run well. IF we get all the
sensors working as advertised and the combo still refuses to run correctly, this is when it will be decision
time: Do you amass the specific tools to tune the black box yourself, or do you join the 0411 club
where there's a lot more knowledge/activity in the area of self-help custom tunes? The point is, what you are
trying to do is not impossible, but there will be a learning curve to be sure.

It's late, and that's all I got for now. More food for thought than answers to be sure, but that's
how these things get going.

Cheers --

 

[Dlipp_]

Hello again Dlipp_,

Let's see, since you're driving a '97 I need to switch to the Vortec perspective, offer a few bits of info, and ask
what visibility you have regarding the live data from your VCM, in that order.

First things first. For some reason I initially got the impression that your engine bay was under TBI control. My bad
for not double checking, but by the same token the ability to access live data (and eventually tuning this yourself)
will be a lot easier in Vortec OBD2 land versus trying to set yourself up for the same capability in the OBD1-era TBI setup.

OK, normally with hard starting the first variable we need to validate is the correct fuel pressure. According to your
parts list, both the fuel pump & fuel filter have been changed out, and I'm assuming that the ground connection
were brought back to new condition while the pump was replaced. (G404?)

But what I found interesting is that by disconnecting the MAP sensor your starting issues disappear? According to
the '97 Factory Service Manual, the MAP and the TPS sensors are used to calculate fuel delivery. And *if* the VCM
detects a malfunction in the MAP circuit, it will substitute a calculated value for the MAP value so acceptable
performance is maintained:

(taken from the '97 Factory Service Manual)

You must be registered for see images attach

From reading your original post, I know that the MAP sensor has already been replaced, so
here is where Mr. Schurkey would want to know what MAP readings your VCM is reporting.
(Problem in the wiring harness, bad out of the box new sensor of dubious parentage, etc)

This would be the next logical step, but for the moment let's assume that you don't currently
have access to live data like that. Instead, let's focus on the starting behavior for right now.

If I interpreted what you wrote correctly, you are saying that the first start is always good,
but subsequent starts are long cranking affairs unless you slightly depress the gas pedal?
(With the MAP sensor connected.)

But if you disconnect the MAP sensor, the starting is always good?

Coming at this from a slightly different direction, if the timing at idle is currently at 36° at idle with
the MAP sensor connected, does it change back to the tuner's spec of 26-28° if you were to
disconnect the MAP sensor?

Q: One thing you haven't mentioned is whether or not you have a SES/CEL light illuminated at
any point? Are there any DTC codes being stored, whether or not the SES light is on?

Obviously at this point I have more questions than answers, but hopefully we can sort out what's
going on and start making some troubleshooting progress.

****

By the way, when it comes to ignition timing in Vortec-land, please keep in mind that ALL your initial
(base) timing is set by Mr. CKP, and has nothing to do with twisting the distributor housing position. (See
attachments #1 & #2 for details.)

As for the 'Cam Retard' value, if it was up to me I would rename it 'Rotor Tip to Spark Plug Wire Terminal Alignment' value.
In English, changing the Cam Retard reading will not alter when the spark plug will fire
inside the combustion chamber. On the other hand, if you get the Cam Retard far enough out of spec,
you may end up with the wrong spark plug terminal closer to the rotor tip than the correct one...and sending
sparks down the wrong spark wire to the wrong cylinder is obviously a sub-optimal tuning strategy.

So, please leave the Cam Retard set to 0°, and let's instead figure out why your timing isn't where
your tuner expects it to be. And no better place to start than to figure out why your engine is
starting better with the MAP disconnected.

If you reply back with the status of your DTC codes, your SES light, spark timing difference (if any) at
idle with your MAP disconnected, and what you are currently using to read data from your computer,
then maybe we can start to tighten up the problem definition.

One last thing. I'm assuming that you are running a black box VCM with a known-good custom tune from
Black Bear, and once we get all the sensors verified good that the truck will run well. IF we get all the
sensors working as advertised and the combo still refuses to run correctly, this is when it will be decision
time: Do you amass the specific tools to tune the black box yourself, or do you join the 0411 club
where there's a lot more knowledge/activity in the area of custom tunes? The point is, what you are
trying to do is not impossible, but there will be a learning curve to be sure.

It's late, and that's all I got for now. More food for thought than answers to be sure, but that's
how these things get going.

Cheers --

Thanks for the knowledge!

long cranking affairs unless you slightly depress the gas pedal?

Yes

But if you disconnect the MAP sensor, the starting is always good?

Yes it goes into high idle for a couple seconds

does it change back to the tuner's spec of 26-28° if you were to
disconnect the MAP sensor?

That I forget, but I swapped to my old computer to test, and the timing was still the same

SES/CEL light illuminated at
any point? Are there any DTC codes being stored, whether or not the SES light is on?

Random misfire only. With the MAP plugged in

Mr. CKP, and has nothing to do with twisting the distributor housing position. (See

When I first took the truck for a test drive it threw a crank sensor code, I replaced it with a gm genuine one, code went away. Is that the issue?

I'm assuming that you are running a black box VCM with a known-good custom tune from

Yes you are correct, black box

Any other info you need lmk

 

[peezy555]

I'd like to say, don't mistake the character of all the regulars here, they're all great people with lots of information to offer. The way you respond to people has a lot to do with reactions, things compound, hence people getting a little snippy. Everyone is here to help, just remember that. There are bad apples almost everywhere, but not here. Those guys get flamed until they see their way out. Like @peezy555. Lol. Take him as an example of what not to do here. Bottom line, respect everyone here and everyone here will respect you. And listen closely to the old guys (I jest), there is not enough time in this world to learn what some have to teach.

Well said I been in Alaska working guys no service but am home and got 30 notifications ha

 

[Road Trip]

Thanks for the knowledge!

Yes

Yes it goes into high idle for a couple seconds

That I forget, but I swapped to my old computer to test, and the timing was still the same

Random misfire only. With the MAP plugged in

When I first took the truck for a test drive it threw a crank sensor code, I replaced it with a gm genuine one, code went away. Is that the issue?

Yes you are correct, black box

Any other info you need lmk

Greetings Dlipp_,

To be perfectly candid, the more I've looked into your situation, the more flummoxed I become.

I should explain, for normally the tougher the knotty problem the more I enjoy the challenge & attempting to
unravel it, for this is exactly how you can really figure out what you know vs what you need to learn more about.

But here's the issue. Using an electrical analogy, IF I was trying to help someone on the west coast troubleshoot a
factory wiring harness that had been hacked up by the PO, no problem. The GMT400 owner and I would both open up
the relevant FSM both here & there, and together we would work our way through the problem, step by step, using shared,
known-good documentation.

On the other hand, let's say that the hacked up vehicle in question had at some point been 'upgraded' to a Painless
(aftermarket) wiring harness. Because the FSM no longer matches the vehicle, there's very little I can do to help, especially
remotely. (Note: I'm not saying that Painless puts out a bad product -- I'm actually a satisfied customer from way back. See #1 attached.)

****

In your situation, the custom tune in your Black Box pretty much nixes most of the troubleshooting I could bring to bear.
Especially remotely, for you would need to invest in a scan tool that does a good job of recording live data, and we would be
chasing fuel trims, spark timing, knock counts, etc. Don't get me wrong, for I think your hard parts choices are well within
reason and there's no doubt that this can successfully sorted out.

But at the very least you will need an onsite mentor who's sorted out something similar in the past. This is the only
way to get enough bandwidth involved so that this becomes a fun weekend master class in tuning vs. a thousand reply
exercise in frustration. On the one hand this site has the capability to share so much info that we would have scarcely
believed back in the daze when a 9600 baud dial-up was the cat's meow. On the other hand, there's a limit to how much
remote troubleshooting can bring to bear, even with text, photos, and (Youtube) videos for sharing the mysterious bits. (!)

Assuming you can't scare up an onsite mentor, Plan B would be to get the Black Bear guru directly involved? Does he
offer this service? No doubt after an afternoon under the hood he should be able to get your powerplant all dialed in?

The backup plan to Plan B would be to consider swapping to a 0411 computer and make use of all the documentation and
hard-won experience that's scattered over the interwebs.

****

For anyone else reading this as part of their research into optimizing their powerplant, please realize that using an engine bay
issue to justify an upgrade is not necessarily a bad thing. But starting with a bad engine bay (wiring, fuel delivery, cooling problems,
etc) ...just installing a hotter engine on top of that plus a custom Black Box tune (that you can't modify yourself) ...and *then*
trying to get it all to work is like trying to take a broken-down vehicle and attempt to push it down the road using a HD rope.
Lots of fussy unanticipated frustration awaits.

****

On the other hand, if you have a bad engine bay & you decide to use this as justification to perform an engine upgrade,
my suggestion is to perform the upgrade in this order:

* FIX everything in the engine bay that's wrong. Texas style cooling system, replace the funky factory quick release for the heater hose,
fresh engine oil cooling lines, etc. Add high quality gauges for those systems that keep you up at night. (Oil pressure, coolant temp,
trans pressure & temp. NOTE: Some folks will require more gauges than others in order to feel 'safe'. (See #2 attached. :0)

* Upgrade to the 0411 computer that YOU can tune. Using the original engine, make absolutely sure that you know how to
dial in the computer to optimize the old engine. Can you figure out how to flip the Security bit so that is one less thing to
worry about? IF you temporarily short out the #5 spark plug wire, does your scan tool report the correct cylinder is misfiring?
(Bonus points for confirming accuracy on all 8 cylinders.) Will your bidirectional scan tool allow you to perform a cylinder
power balance test by shutting down the injectors one at a time? Can you set your CMP RETard? How about the CKP relearn?
This is often overlooked, and not that many consumer-grade tools can provide this necessary function.

In English, give yourself the gift of accurate live data *prior* to pulling out the long-suffering original engine. This way,
when the new engine with all the spendy bits inside is installed and started up, the engine will have the best chance of
lighting off immediately and performing the all important break in such a way that you minimize the chances of wounding it
right out of the box. Or toasting the cats because I can't gain positive control over the fueling, or can't suss the misfires,
etc.

To continue the analogy, sorting out ALL of the ancillary support systems in the engine bay using the original engine
as the crash test dummy and only *then* installing the new powerplant is like taking that HD rope and pulling the
broken-down vehicle down the road. Nothing quite like being more successful while simultaneously trying easier.

****

To the OP, please don't take this as my being judgemental. Instead, when you mentioned that you had replaced the
CKP sensor, I was going to ask you if you performed the relearn after, and then while I was trying to figure out which
scan tools/software-simulated scan tools provided the relearn function...I realized that this was a long shot to fix the 'bad
start with MAP connected' symptom...and then ultimately I wouldn't know how/where/why Black Bear changed the
underlying tune, so I would end up chasing my tale. And I've always hated guessing in public, especially when other
people's time & money are involved.

By the same token, I would like to thank you for sharing your situation, for while doing research on possible solutions the
upgrade sequencing for the engine bay of the chore truck came into sharp focus. And I consider this to be a big help in
ensuring that my own (re)build thread will have a strong, powerful finale. :0)

Best of luck sorting this out. And when you get the right talent involved and the problem(s) are all sorted out, please
report back with what it took to get your GMT400 upgrade all wrapped up.

Cheers --

 

[skylark]

When I first took the truck for a test drive it threw a crank sensor code, I replaced it with a gm genuine one, code went away.

Did you do a crank relearn? This requires a scan tool.

Did you degree the cam?

 

[Dlipp_]

Did you do a crank relearn? This requires a scan tool.

Did you degree the cam?

Apparently the 96-97 don’t need a crank relearn. The cam was put it dot to dot. However, getting a hold of black bear is tough. He gave me a tip and it worked some what. Adjusting the throttle blade… so he said that with the big cam, it’s not getting enough air on start up, the pcm is trying to help it by giving it timing. Sure enough I opened the throttle blade up, thing fires right up and I could find tune the spark advance (timing) to the 26-28 degrees it’s supposed to be at. Only down side is it has to idle at 1000 and you can’t hear the chop anymore. (What’s the point of a cam if it doesn’t choppy choppy). I’d say for the most part my issue is fixed. But I would like to raise the misfire detection so I can have it idle around 950 and chop. Start fine and no misfires. Idling at 950 the timing is right around 30. I would think 2 degrees advanced timing (compared to what the desired is) would be acceptable, hopefully.