stumped with 8.1 Vortec issue

[Cam-Shaft]

Hello Gents,

Recently purchased a '91 K5 blazer that was repowered with a 8.1. Overall it runs great and I'm quite pleased with the rig however there is an issue that I cannot seem to track down.

Usually when cold, it starts right up no issues and idles normally. When warm it will often start but idle very low, almost not running at all and when you press the accelerator there is no response at all. The Compushift unit shows that the TPS is working and when I took off the air filter, I can see the intake manifold moving properly.

My fix is usually to shut it off and try again, and usually 10-15 times later it will start and idle normally with the accelerator working again. I am trying to figure out what the correlation is between the low rough idle and the lack of throttle control....

I recently replaced the battery/alternator (problem happened before) and yesterday replaced the MAF sensor as it seemed to fit the problem description.

Warm engine - low idle - no throttle response whatsoever....

Thoughts? Thanks in advance.

[clinebarger]

Stored Codes??

[Captainfab]

Do you have a check engine light? What about a OBDII port to plug in a scanner or code reader?

You might get more responses if you ask a moderator to move this to the LSx swap section.

[cableguy0]

Moving to the ls section.

[Cam-Shaft]

Thanks for the replies, I'm going to try and get it hooked up to a reader this week. Only concern is I am still learning about how the engine swap was done, so I'm hoping there's not a bunch of random codes from parts that are no longer hooked up. I'll get a reading and post it here

Thanks for moving it, I wasn't sure quite where it belonged being the 8.1 isn't technically a LS

Regards, Jon

[Marshy]

Originally Posted by Cam-Shaft

Thanks for the replies, I'm going to try and get it hooked up to a reader this week. Only concern is I am still learning about how the engine swap was done, so I'm hoping there's not a bunch of random codes from parts that are no longer hooked up. I'll get a reading and post it here

Thanks for moving it, I wasn't sure quite where it belonged being the 8.1 isn't technically a LS

Regards, Jon

I suggest reading the codes and take note of which are in. Then I like to clear the codes and wait for them to come in while you drive it (check when engine light comes back on) and read what's in. If you don't have a check engine light because of the swap, you might want to consider buying a cheap code reader and leave it plugged in while you drive it. Then you will see them as they come in. Some of the nicer ones they sell at the auto parts store will give you voltages for sensors that are useful for troubleshooting.

[akdg87]

can you scan coolant temp through the PCM? Sometimes they go bad and cause weird problems. Pull the plug on the coolant temp sensor(the one to the engine PCM) while running and see if it clears up. Also check fuel pressure...

[cableguy0]

Originally Posted by Cam-Shaft

Thanks for the replies, I'm going to try and get it hooked up to a reader this week. Only concern is I am still learning about how the engine swap was done, so I'm hoping there's not a bunch of random codes from parts that are no longer hooked up. I'll get a reading and post it here

Thanks for moving it, I wasn't sure quite where it belonged being the 8.1 isn't technically a LS

Regards, Jon

I put it here because the guys who are extremely familiar with tuning the late model stuff usually hang out in here. Tuning seems to be your issue and they should be able to help you in here much more thoroughly than you would get elsewhere.

['63GENIII]

My limited experience and research with the L29 (somewhat similar to your 8.1) that is going in the '63 has taught me that if the engine is swapped minus some of the factory smog related parts i.e. rear O2 sensor, EGR, EVAP purge valve that these items will need to be programmed out of the ECM / PCM. If not, as the engine goes from (and correct me if I get this wrong) open loop to closed loop after warm up, it will be looking for the missing sensors / devices. If it cannot find them it may go into a "limp" mode since it cannot get feedback from the missing devices. I do know that when I had my ECM reprogrammed, I was asked for all of the modifications and all of the smog related devices that I had removed so that they could be eliminated from the necessary inputs needed for the ECM to make the engine run correctly.

Even with all of the information that I gave to the programmer, I still suspect that there will be a glitch or two that will need to be worked out when I go to fire it.

Do you have the 0411 PCM (2 long connectors) or the Black Box PCM(4 smaller connectors)? There are a few guys out there that can send (rent) you equipment that can log data from the 0411 ECM while you drive. Should make it easier to find out what the PCM is looking for and trying to compensate for if its missing.

My L29 is missing the EVAP purge, EGR, has had bigger injectors and cam programmed in as well as going from a 4L80E program to a NV4500 manual trans. Is it possible that your ECM had never been re flashed?

[Cam-Shaft]

All great things to try, still waiting on some time as work has been busy as we ramp up training. I have no idea on what was done when the engine swap took place. Correct me if I'm wrong, but the fact that it runs excellent when hot and only when shut off and attempted to restart is when the issue presents itself seems like it would rule out most of that... Plus the fact that it eventually starts after enough trys (even while engine is still hot). Looks like tomorrow I'll have a ride off base and be able to pick up a decent scanner that I can leave plugged in over the weekend while I put some miles on it. Again, thanks for all the suggestions. I'll keep all you gents posted.

Best,

Jon

[ls1nova71]

What's your fuel system set up like? In tank pump or external? A lot of what you describe sounds fuel related to me, like maybe you're pump is getting hot and not working as effectively as it should. This usually only happens on external pumps though. Could also be that the wiring to the pump is not good enough and can cause the same thing. When your fuel pressure gets low, the throttle will sometimes feel as if it isn't working, so I would get a fuel pressure gauge and check your pressure when cold and also when it starts messing up and see what you've got.

[cg285]

we get a lot of limp mode complaints on the 8.1 when the battery voltage drops too low when cranking. it confuses the pcm which in turn sends it to limp mode