TBI 454 idle problem

[Godholio]

I recently bought a 95 K2500 Suburban. The AC blew hot but the compressor looked to be engaging well and the engine sounded great, 4L80E shifted well, and the interior was in surprisingly good shape (dash isn't even cracked!). The engine was warmed up when I test drove it, but the seller warned me that it idled weird when cold, and usually stalled right away once or twice before running better. Good enough for the price!

A can of refrigerant and the AC blows cold enough for the moment, the barn door latch is fixed, and I replaced the tires that were almost old enough to vote, so now I'm trying to sort out the idle.

On a cold start (I'm in Vegas so that just means the engine isn't at normal temp. It's still 90* by 9 am) the truck will stall as the seller described. It'll catch then die. After one or two stalls it'll finally stay running, but the factory tach fluctuates between 500 and 1000 rpm. Doing anything to drop the rpm (putting it in gear or turning on the AC) and the rpm just falls off and it stalls. Driving is perfectly fine. Engine sounds and responds as it should. Coming to a stop, there's about a second delay after actually stopping before the rpms start to fluctuate again, but it usually won't stall. I've only had it out for a couple of drives long enough to get up to temp, but it seems that as soon as the temp needle gets just shy of 210 (center) the idle issue goes away. I was idling in a drive-through last night when it just stopped fluctuating and settled at about 750-800 according to the tach...right at 210*. I've never seen the CEL on, but I didn't check codes prior to disconnecting the battery. Of course there's nothing now.

So far, between this problem and just catching up on a tune-up that I figured a 244,000 mile truck would need, I've done:
plugs
wires
ignition coil
PCV
PCV vac line
IAC
MAP vac lines (most of the vac lines actually)
vac plug on the back of the TB (I don't know how, but it was split)
fuel filter
battery
removed/cleaned TB, even though it surprisingly didn't really need it
alternator just tested ok at autozone

I haven't replaced the TPS because I figure if that were the problem I'd have a light and code by now and I'm not sure that makes sense since it looks like an open-loop issue. I didn't mind replacing the coil because even though it tested fine with a multimeter, it could've just been weak since it was original. My next step is to try cleaning up the big 3 grounds (and the dash ground, the voltimeter dances with the blinker), and I've got an ALDL-USB cable on the way but it's going to be a few days.

So, what am I missing? I feel like I've done the obvious solutions, and I'm not sure where to go next until I can get some data logged and compare the numbers to what they should be. Speaking of, what's a good resource for factory settings?

Here's a short video (with sound, but it doesn't add much). The truck is doing this on it's own, I'm not touching the pedal.
https://i.imgur.com/Cb1ChFp.mp4

 

[HotWheelsBurban]

Throttle position sensor doesn't always throw a code. My 99 vortec 5.7, it just went crazy one day on startup when the days before, it was running fine. The idle was fluctuating wildly with nothing close to the cable or gas pedal. Was told that it was the TPS,changed it, no more idle fluctuations. The old one was undoubtedly the original one, and it was very loose inside ( the pin that goes into the linkage was wobbly as hell),whereas the new one was tight.
And it did not throw a code at all......

 

[RawbDidIt]

Could be the TPS as hotwheelsburban pointed out. TPS will affect open or closed loop, so you're probably right that it isn't since it doesn't affect it after warm up, but it still could be. More likely it's MAF, vacuum, or fuel related though. Check fuel pressure and vacuum before proceeding, couldn't hurt to give the MAF a good spray with some MAF cleaner and reinstall.

Sent from my SM-N976V using Tapatalk

 

[PlayingWithTBI]

Did you calibrate your TB after replacing the IAC? Here's a .pdf that shows how to set your base idle, IAC, and your TPS voltages. Plus as @RawbDidIt suggested, check your fuel pressure, s/b 11 - 13 preferably closer to 13.



BTW, there isn't a MAF on the TBI engine, it runs strictly on speed density (MAP/RPM).

 

[RawbDidIt]

Did you calibrate your TB after replacing the IAC? Here's a .pdf that shows how to set your base idle, IAC, and your TPS voltages. Plus as @RawbDidIt suggested, check your fuel pressure, s/b 11 - 13 preferably closer to 13.



BTW, there isn't a MAF on the TBI engine, it runs strictly on speed density (MAP/RPM).

Effin A. As usual thank God you're here to school me on the differences between vortec and TBI engines lol. The MAP sensor can cause idle issues as well, but check vacuum first, as it may be functioning correctly, and the vacuum leak is the cause as opposed to the MAP. If fuel pressure and vacuum don't give a clear picture as to what's wrong, get your hands on a scan tool that reads live data and confirm your MAP is sending the correct information to the ECM.

Sent from my SM-N976V using Tapatalk

 

[Godholio]

Did you calibrate your TB after replacing the IAC? Here's a .pdf that shows how to set your base idle, IAC, and your TPS voltages. Plus as @RawbDidIt suggested, check your fuel pressure, s/b 11 - 13 preferably closer to 13.

I set the IAC according to the factory service manual, but can't do the full calibration until I get the ALDL cable which is on the way. Also need to find which program will read my ECU...I'll be looking into that later today.

As far as fuel pressure, I've got the RV Morse adapter and gauge, but I have to say I'm extremely disappointed in it. Looks like I'd have to loosen a clamp somewhere down the fuel line to get it to flex far enough back from the throttle body to give the adapter room, but I think it would actually also have to be bent. Not happy about that. Not that it matters, because the gauge itself is too big to fit. Or the hole for the gauge to thread in is too close to the rim of the throttle body, however you want to look at it. I'd have to find another fitting so it could stand off further away from the adapter. So the jury's still on how I'm going to tackle that. After replacing the fuel filter I've had quite enough of laying on the pavement in the Las Vegas summer. As a 7.4, the fuel pressure should be 26-32 psi.

I can't find any more vacuum leaks, but I'll keep looking.

 

[PlayingWithTBI]

Looks like I'd have to loosen a clamp somewhere down the fuel line to get it to flex far enough back from the throttle body to give the adapter room, but I think it would actually also have to be bent. Not happy about that. Not that it matters, because the gauge itself is too big to fit. Or the hole for the gauge is too close to the rim of the throttle body, however you want to look at it.

I'm not sure on a 7.4L but, on my 5.7L all I had to do is bend the MAP sensor vacuum tube down a little and drill an offset hole in the base for the adjustment screw - no biggie.

You must be registered for see images attach

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Yes, in order to get the fuel lines to reach my Edelbrock 3704 intake I had to loosen up the clamp on the bell housing (you can reach it from underneath) to get some more slack.

 

[PlayingWithTBI]

Also need to find which program will read my ECU...I'll be looking into that later today.

With TunerPro Rt, if your PCM is a 7427 you can use $0D mask.

http://www.gearhead-efi.com/Fuel-In...o-Definition-Files-Wiring-Diagrams-Tuner-Info!

 

[Godholio]

You must be registered for see images attach

Yes, in order to get the fuel lines to reach my Edelbrock 3704 intake I had to loosen up the clamp on the bell housing (you can reach it from underneath) to get some more slack.

In that picture you can see where my problem area is...the TB is wider on a 7.4 than a 5.7. That gap between the rim of the TB and the gauge? That doesn't exist. The gauge doesn't fit. I'll have to grind down at least 1/16" on one or the other to make it work.

And you've got braided steel lines...a lot more flex than the steel tube I've got (but much nicer).

With TunerPro Rt, if your PCM is a 7427 you can use $0D mask.

http://www.gearhead-efi.com/Fuel-In...o-Definition-Files-Wiring-Diagrams-Tuner-Info!

Excellent, that's one I was looking at; yeah it's a 7427.

 

[Godholio]

I'll just post updates in here I guess. Back when OBD1 was current I was a teenager and didn't have the money for any of this stuff (none of which was free!), so I'm learning about this datalogging/tuning for the first time (and a lot of the data is on sites that no longer exist it seems). I'm still waiting to be approved at gearheadefi, so I only have the bin files not the others. Still waiting on the cable, so all I can do at this point is read the internet. It look like $0D is for a 454/4L60E, the $31 and $0E are 454/4L80E and the $31 matches my BMHM memcal. I think.

What I really want to do at first is see what the factory specs are compared to what my truck is reporting. Later I'll get into really trying to tune. I kind of like the idea of having a couple of chips ready to go for different uses.

Edit: Found the other files on Tunerpro's site. There's uh...a lot more tables than I was expecting.