Rebuilt 454 engine (slightly modified) Rebuilt tranny - stalls out in gear at idle - help...

[Kraig]

I picked up a 1997 K2500 Suburban w/454 in it that the previous owner had rebuilt, he drove it a few months & when the tranny took a dump I was able to buy it for a song & dance basically. I took it to my favorite transmission shop. (that was the deal with the wife, I couldn't do the tranny work) He put in a GM Goodwrench factory rebuilt 4L80e. A little bit of brake work and I have it on the street. It starts up great, but seems a little "cold blooded" at start up for about 15-30 seconds. (I know weird to call a FI engine cold blooded) but the problem I'm running into is that when I put it into gear it stalls. If I hold the brake and get the RPMs up to about 1000 then it doesn't stall. Once I'm driving it is usually OK but again at stop lights it wants to stall out about 1/2 of the time.

Unfortunately the "kid" I bought the burb from didn't know much about the engine other than they put in bigger injectors, a cam with 502 lift & valves to match, holly ignition, cold air intake, but they left the stock exhaust manifolds on. They also removed the cats on this one. I've lost touch with him so I have no good way to get further info on the engine like we originally talked about. I've hot rodded a few cars in my day but never really messed with a FI computer controlled engine. I can tell it needs some tweaks because it smells a little rich but it has been over 30 years since I've played with a carb'ed engine that is easy to play with.

I'd love some suggestions before I take it into a mechanic and at least try to eliminate the simple things like plugs, wires, cap, etc. I've replaced all 4 O2 sensors since one was smashed and one was missing. I really need it to run good at low RPM because my primary use for this 4x4 is for hunting and driving down rough log trails. My secondary use is towing my XJ 4x4 on a car trailer for off-roading.

HELP

 

[Erik the Awful]

Check for vacuum leaks first. If you have a good tuner nearby, I'd consider getting it tuned, but you need to be sure the problem is the tune and not something else before you take it to them.

 

[WillB]

Does it do it once it is warmed up or only when its cold? I'm fighting a very similar problem on my 99 454.

 

[Kraig]

WillB - It does it both cold & warm.

I did a vacuum hose check and right under the throttle body there was a boot that was off that was sucking air. That helped a tiny bit. Looking further I see that on the drivers side there is an old school valve cover breather installed where I'll bet there should be a PCV valve. I should have seen that breather right off and questioned it on a computer controlled engine.

The first picture is the piece that wasn't hooked up.

The second picture is my drivers side w/a breather instead of PCV valve.

The third picture is one I found on the internet of a 7.4 liter that I believe is the correct PCV set up I need. However since the intake port is blocked off I don't think that is contributing to the stalling when putting it in gear.

Adding that it idles at about 1000rpm when cold and then drops to about 700ish once warmed up. (might be a bit lower, don't trust the factory tach w/300K miles on it)

 

[gbiddlec]

You are correct on the third picture being the correct PCV setup. The middle section is a pre-bent plastic tube so you'll need the hole setup (tube & valve). Just spitballing, but could this be a throttle position sensor issue?

 

[honkon]

So exactly what type of ECU does this engine have? You said Holley fuel injection?
Does the OBD connector under the dash have power and communication with ECU?

 

[454cid]

I don't think the rubber elbow/fitting on the passenger side is available anymore. The PCV hose is NLA. I used a piece of bulk emissions hose.... low pressure fuel line, vent hose... several names.

 

[Kraig]

UPDATE: Picked up a PCV installed it with some gas line since the molded rubber part isn't available any more AND fingers crossed it seemed to be doing even better. I drove it for about 30 minutes to get to the mechanic I'm using to rebuild the front end and when I got there it didn't stall when I stopped. Put it into park, into reverse, back into drive and it was maybe a little low on the idle but didn't stall out. I'll be in awe if all it ends up being is one vacuum hose on the passenger side and replace the old school breather with a PCV.

honkon: it has a factory ECU, unknown if it has a tune in it or not. The injectors are suppose to be "bigger" with up to 30% more flow but unknown what ones. The part that is Holley is just the ignition coil, not the fuel injection system.

gbiddlec: I asked the mechanic to do a once over on the engine just to check for anything that concerns him. Check the timing, codes, etc as long as he is in there...so if there is a TPS problem he will let me know.

Kind of sucks having to take it to a mechanic but my company has me going out of town for two weeks so that takes up all of my garage time between now and hunting with my dad & brother. I'll have about 1-2 weeks to finish figuring out the electric 4x4 weirdness and to get it aligned and hope everything works good enough for a 5 hour drive to the farm we hunt. I'll update on anything I find out. Running good is first priority, then it will be time to tinker. Maybe 4.56 gears & disk brakes on the back. Rip out all the nasty carpet, clean it all up. Come up with a grill that looks a little mad max maybe, a rear bumper to match the front and then new paint & an overland rack up top. Should be fun.

Thanks for all the help so far.

​

 

[Kraig]

Been super busy, forgot to update where I'm at with this. The good, the bad, & the ugly coming up...

The bad: Had to pay a mechanic to sort it out. Love Anthony (the mechanic) so far! Had to replace the over sized injectors with stock flow injectors. That plus the vacuum leaks was a HUGE part of the problem(s). Still have a problem with the idle control valve. (see "the ugly")

The good: Raw gas smell at idle and when accelerating, fixed.
Smoothed out revving from idle to 3500 rpm (and probably higher but haven't gotten on it that hard).
Idles fine after it is warmed up.

The UGLY: A previous owner screwed around with the computer & has the idle control valve set in such a way that when it is cold, the engine wants to stall out. The mechanic doesn't have programming tools so I'm kind of screwed for now. (new post or search coming soon) The mechanic modified a idle control valve so the truck runs good once warm & is just a bit touchy when it is still cold. Hats off to him for getting it that far wo/a programmer gizmo. (and he threw in a brand new idle control valve I can put in once the tune is fixed) Right now I have a 1997 fuel injected engine that acts like an 1967 454 when its cold...but I can use it for now and move on to the 4x4 components.

Moral of the story: Make sure you don't buy a non-driving "just rebuilt" 454 that's been modified, you might end up with a slight headache.

 

[Kraig]

Next update in the attempt to get this unknown spec 7.4 running well.

Last time I shared what my mechanic did for me with bringing the injectors back to stock & the trouble with the IAC (idle air control servo gizmo in the intake runner).

This installment of as the Chevy Turns...I picked up a bone stock PCM from FlagShipOne https://www.fs1inc.com/1997-chevrol...omputer-pcm-ecm-ecu-programmed-plug-play.html for about $151 on sale. They were pretty easy to deal with but they take longer to ship the PCM than advertised. Ordered it on 4/8 & it shipped on 4/22 getting to me 3 days later on 4/25. Super easy to disconnect the original PCM, plugged the new to me PCM in, replaced the ICA with a new one, & it fired right up. At first it didn't want to idle perfectly but it did idle where before I had to wait until it was fully warmed up to hope it would idle. Over a the next few short drives it seems that the computer was learning but now sometimes it will take a few cranks to start where when the PCM was new it fired right up.

All that to say, I probably should have just sent my stock PCM to Black Bear Performance to get a "traditional tune" service for about $300 and hope that their experience will get me a tune that work with an unknown spec 7.4 build. (where I know the pistons are new, the cam isn't stock, and there are a few other non-factory / non-stock changes BUT the actual specs are all unknown to me) NOT a good way to get a performance tune. IMHO.

With that said, I'm seriously considering getting one of the 0411 PCM(s) sending it to Black Bear to put a "traditional tune" into it and then ordering the "scan cable tune" where they send me a scanner that I hook up to collect data off my engine specifically, then send the PCM & the scan tool back for a custom tune. Should only cost about $350 plus shipping both ways. The trick is finding the right PCM and then finding the correct PIN out to change the harness to fit the new PCM. All I know is the 0411 is a better, faster, & more tunable PCM and allows me the scan tool tune option where as the 1997 PCM doesn't even give me that opportunity. Some 1998-1999 PCM will work with the scan tool that Black Bear uses but still require a pin out change so if I'm going to do that, going with the 0411 is the better route from my current understanding.

Bottom line, as of right now the suburban runs with a factory tuned 1997 PCM. It has a slight hesitation from idle to "go" sometimes but towed my car trailer w/a Jeep XJ on it like it wasn't even back there. I just think I want it running better than it is right now and it is worth 350 bucks or so to get it that way. No idea on MPG as it sits but I should do that before taking it apart again. More info as I progress.

FYI: LexTech is about AWESOME!!! I sent him the the request for the 411 pin out & boom it was there in a matter of what seemed to be only minutes! Hats off for speed of delivery. (that was my experience, your mileage might vary)