Stumbling after start up
 

1985 C10, 2WD, LWB

2003 Tahoe, 5.3, DBW/non-DOD/Flex-fuel/DBW. Return fuel system, 4L60E, New 1987 baffled tank, New sending unit and filters, New Fuel Pump in-tank AC Delco EP381 61.6 PSI., Stock Plugs-new wires. Flex removed during reprogramming.

Symptom/Condition: Upon initial driving, engine runs rough. Bucking, stumbling, etc but not trying to stall. Just severe loss of power. No Service Engine Soon light. No codes. No backfire. At idle, you can’t tell it’s happening until first acceleration. Give it lots of throttle or a restart and it goes away. Voltage seems normal. Gas at a top tier station, gotten several tanks there before. (In multiple cars) had ¾ tank of gas during issue.

---So after 2000 trouble free miles on my swap, I have developed an issue that comes and goes.

Drove the truck Saturday afternoon, no issues whatsoever. The issue started Saturday Night. Truck parked flat in bottom of driveway. Started up quickly and on the first try as always. Sounded normal. Tried to drive up to the top of the driveway and it was bucking and stumbling. When I got to the top, I turned the truck off and waited 30 seconds. Upon restarting, the truck found an idle and drove with plenty of power, like normal for the rest of the trip. Went and came home without shutting it off.

Sunday came and the first drive to my buddy’s house, everything was normal. Parked the truck in his driveway, nose up. Starting up to leave was an hour later and the problem started again. Drove down his street, pulled over and did the whole shut off/restart and power was back. All was good again.

Next stop was about 20 mins. At O’Reilly’s. Upon restarting, I did not notice the problem immediately as the truck was parked nose down and I put it into gear and coasted to the exit. The store is on a 4 lane highway so I pulled out when clear and the problem was back. Bucking, stumbling; almost like it was missing but like 4 out of 8 cylinders were not firing. Planned on pulling over for the shut off/restart when traffic started bearing down on me. As not to get run over (and because I was getting annoyed) I dropped the pedal and the truck took off and got up to speed like normal. It found it’s happy self and responded the way an LS should. Turned off the highway and it was purring like a kitten for the rest of the ride.

Ran two more errands and parked in a flat parking lot both times. The problem came up on one of the times. A quick step on the throttle and it issue was gone for the rest of the ride.

No codes stored in computer. Doesn’t happen every time, but is getting more frequent. Thought it might be choke issue.

Any ideas? I am worried as this is our second car and do not need my newly licensed daughter having a problem out on the road.

I can do small diagnostics at night, but by the time I get home from work with the time change, it will be dark. So bear with me on replies. More in depth diagnosis would wait until the weekend.

Thanks

Re: Stumbling after start up
 

I had some problems with my 6.0 swap similar to yours. my problem was with the maf sensor so that's what I would check that first you can tap on it with it running and if it stumbles you will know its the maf sensor if that make no difference try unplugging it with it running and see if its idles smooth with it unplugged . after that if no luck check to see if you have any vacuum leaks I have seen a few intakes that will get cracked from being removed and installed .also are you sure the engine has the dod ?I was thinking that didn't come out until the 07 modles up

Re: Stumbling after start up
 

MAF could be the issue. Also check idle air valve. I've had startup issues come and go with a failing IAC. As 4x4k30 said, you can unplug the sensors to see if things change. I believe unplugging the MAF causes the computer to default to a rich fuel map. Also remember to disconnect the battery for about 10 to reset the pcm after changing to a new sensor.

Re: Stumbling after start up
 

DBW trucks dont have an IAC/IAV, they use throttle position to modify idle control. Unplugging the maf forced the truck into speed density and uses primary VE for fueling instead of trims. Unplugging it would help you DX issues with the maf or 02 sensors.


Have you checked fuel pressure when the stumbling occurs? You mentioned 61psi but I'm guessing thats at idle under no load.

Re: Stumbling after start up
 

Oh yep, you are correct. Skimmed over the specs initially and missed the by wire. With the electronic throttle body, there is a relearn procedure for that. Not sure what warrants a relearn though.

Re: Stumbling after start up
 

Thank you all. You guys are awesome! I do have some follow up questions.

Just so I have this correctly, I unplug the MAF when it's stumbling and if there is a change, it's working correctly? No change it could be bad?

Tapping on it while running might also produce the stumble and indicate a bad MAF? Right?
(this one is expensive, I don't want to buy one and not need it)

The relearn is for unplugging the MAF or the TPS? That would be the disconnect the battery or another procedure?

Finally, what is "uses primary VE "?

I assumed it was DOD as the reprogramer said he took it out. maybe was a canned email and it wasn't there in the first place.

I have not yet checked fuel pressure but plan on it this weekend. the 61 psi was in the selling spec, thought it might be good info for diagnosis.

So far this week I cranked it up Monday and Tuesday night, drove up driveway and all was good. Wednesday morning, the problem was back but again, quick blip of the throttle and it was good for the entire trip.

Re: Stumbling after start up
 

You need to have the engine running and leave the maf sensor hooked up then at idle tap on the maf sensor and see if you engine stumbles any when you tap on it, ifs it bad it will make the engine stumble when tapping on it . then if that's ok . just try to unhook the maf sensor when the engine is running to see if stumbles when you plug it in and out . also I have bought 2 maf sensors on ebay for 25 each and believe it or not they have made the engine run fine on my 79 6.0 and my 77k3500 with a 6.0 they stopped the maf sensor code p0101...https://www.ebay.com/itm/Mass-Air-Fl...72.m2749.l2649

Re: Stumbling after start up
 

If it were me I'd do the following:

- See if it runs better wit the MAF disconnected, and if so, you have an air leak or a bad MAF mounting position
- If the above doesn't fix it, check for header exhaust leaks

Re: Stumbling after start up
 

Let us know your progress.

Re: Stumbling after start up
 

Ok, so this evening started truck. Everything was ok. No stumble. Drove top of driveway, plenty of power.

Tapped on MAF and NO change. Unplugged MAF and only a slight stumble and lower idle. No check engine light. Plugged back in, restarted and purred like a kitten. Still no check engine light. Only time for check engine was during bulb check.

Father-in-law drove it today. No issues.

Running 2010 camaro exhaust manifolds. No leaking, sounds quiet underhood.

When it happens, it's like the choke on a carb is stuck and carb is loading up. A little "half throttle" clears it right up. Weird.

Re: Stumbling after start up
 

Look at the purple and yellow wires coming out of the throttle body. Give them a very gently tug n shake and inspect them for cracks. Damaged DBW wires can cause horrid intermittent problems.

Re: Stumbling after start up
 

Thanks for helping. This is a little frustrating as it doesn't always happen.

Here is the Latest: This morning I started it and it was idling rough. Immediately pulled the MAF plug and there was NO change. Plug/Unplug several times, no change. No Check Engine light. Tapped on the MAF with no change either.

I am ready to condemn the MAF but wondered if it being in Choke mode would have anything to do with it not changing when the plug was pulled? It had only been started for less than a minute.

Another thing was that I left it running after I testing the MAF plug as described. Started driving and it was not a peppy as normal. Kinda held back but not stumbling. Got to a red light and did the shut off/wait 30 secs/start up. After that, Smooth idle, good power, no hesitating or stumbling.

Will have a better chance at testing the fuel pressure this weekend. Also more a chance of several errands that will be the true test of the MAF unplug/plug test-both with and without choke. Will also look at the throttle body wires.

Re: Stumbling after start up
 

Un plugging the battery will do nothing except re-set the clock. These vehicles use electronics with EEPROM's. Is the MAF sensor mounted at least 8-9" away from the throttle valve? Is the air-flow grid installed in the MAF sensor? The MAF sensor will have air temp built into it. This to is important. The PCM uses this information compared to engine coolant temp to change from cold start to hot normal start. Make sure a scan-tool reads this also as what ever dash you have is using a different sensor. When un-plugged, it will read -40 with key on, engine off. Should be close to dash temp when running but check with IR temp gun at the head.

Wires are important. Intermittent problems can be caused by un-soldered splices or as said, wire connectors pulled hard with strain on them. Also, if possible, use a wire terminal (Like the 150 metro-pack) to check for terminal fit. During the swap, a wire terminal can be spread and have poor continuity. Use an electrical terminal cleaner and but some dielectric grease sold at home improvement stores and boat shops. Put it into the connector of everything under the hood...even you light bulbs. But please check terminal fit.

Having an injector stick open will cause similar issues but would turn on CEL. On OBD-II, there are type "A" codes that turn on the CEL, the first time a test is ran & fails. Type "B" codes will only turn on CEL after 2 test have ran and failed.

If your problem is mostly at startup, make sure the PCM is turning on the fuel pump for 2 seconds. Wiring is key to fuel pump relay from PCM.

If you had access to a TECH2, you can build year, make, model and engine...then set the scan-tool to record using manual trigger. It will capture all engine data before you start, while cranking and after cranking. You can get valuable information from reviewing this. Since your problem comes and goes, it may require several attempts to capture what you need, but golden info.

Key on, engine off, look at MAP sensor readings. It will set base altitude and read about 98 kpa. When running, the MAP sensor will drop to 28 -36 kpa during this time depending on camshaft. The MAP is overlook during key on, engine off all of the time.

Checking fuel pressure is a basic starting point, but if it runs lean or rich once engine is up to temperature, for a set period of time, only then it start adjusting base upon O2 sensors.

Re: Stumbling after start up
 

Did a little more diagnostic work this weekend.

Tested fuel pressure. Had 41 PSI at idle and 52 PSI with the vacuum line pulled off the fuel pressure regulator. Could not check pressure under load as my gauge does not have enough hose for me to see it while driving. I did put a finger on each of the 8 injectors and they were clicking like normal. (BTW, word of warning, do not try to connect a fuel pressure gauge while engine is running, IT SPRAYS EVERYWHERE)

Then went to the local pull a part with my buddy and found a MAF. Installed it, but quickly set a code. So that one is no good. I`m back to the original one now. Still not ready to rule it out, but getting more confident with it.

Did check the wires at the throttle body. All look good. moved them around and pulled slightly but no stumble. Then decided to disconnect the plug and it really ran rough. It didn't run like that when I was just moving the wires around, so I think the throttle body can be ruled out.

Also checked for a loose plug wire but all looked good and tight.

Still doing it sporadically. If I get home before dark, will check some more.

Re: Stumbling after start up
 

Interested in final solution. Following.

Re: Stumbling after start up
 

That makes two of us!

Drove it for a quick run this morning. Thought I might have felt a stumble/Hesitation, but not 100% sure.

Stay tuned.

Re: Stumbling after start up
 

One thing I forgot to mention is that I noticed a squeak in the accessory drive belt. Didn't think much of it because it was a new belt. But while doing some of the above checking under the hood Sunday, i noticed the belt tensioner jumping back and forth under idle.

Took the belt off and found the tensioner pulley good but the idler pulley to have a slight drag to it. Replaced that and the tensioner has stopped vibrating back and forth.

Drove it this morning to take the dog to the vet and both ways, no stumble/hesitation. I think I may have been chasing a problem in the engine when it was the pulley. My guess is that at idle, the tensioner would vibrate (stumble) and once given some gas would overcome the drag, (hesitation) and the tensioner would stop vibrating then the truck would drive normal.

I am still reserving judgment pending a few more test drives.

Re: Stumbling after start up
 

After years of dealership experience, I have seen bearings lock up on alternators or idler pulleys and with a 6 row serpentine belt lock up engine at idle stops, then towed in for with a no crank symptom. It is amazing how much that belt can grab & hold.

Serpentine belt can have hundreds of cracks and most shops will try to sell one based upon that lone, but only need replaced when they stretch.

The "Live" belt tensioner's have marks on the housing and arm on it for maximum stretch. Clean it off and look for the marks first. The idler pulleys are subject to wear at the nylon pivot or the pulleys bearing. Many pulleys take the front alternator bearing "203FF" bearing, which can be replaced.

Always check the belt, tensioner, pulley or bearing and replace what is needed. Use a long 2' screwdriver and place it on the bolt or housing, next to the rotating part. This is the cheap & quick way to hear noise.

I have seen sprays for belts while some like WD40, but none work or last. Simply clean grooved pulleys with tiny sand particles caught in the grooves when cleaned fix most of belt noise issues after check the "live Tensioner" for movement. Yank the belt, then see if the tensioner returns fully to the tight position. If not, replace it.

Good catch.

Re: Stumbling after start up
 

I have experienced stumbling at startup as well. When at low idle I can hear the idler making a clicking or knocking noise. It happens mostly in reverse.
So you didn't change the tensioner, just the idler pulley?

Re: Stumbling after start up
 

Yes, just the idler. The tensioner was replaced as part of the install. (My engine my donor came out of took a massive hit to the front. the Airbox, fan and tensioner were broken. Even the starter was broken!) I still have avery small squeak, so the long screw driver to ear test will happen this weekend.

I did take it for a small drive last night, no stumbling or hesitation. A couple more test drives like these and I can probably put this issue to bed!!

Re: Stumbling after start up
 

Following this.

Re: Stumbling after start up
 

I have been following a LS1tech Thread in which guys are reporting a chirping from the belt. They say that a gatorback belt or equivalent continental "quiet "technology belt has solved the issue.

Re: Stumbling after start up
 

While the "Gator Back" belt may solve noise issues, the lateral cuts allow for quiet operation while having alignment issues with pulleys on the accessories. GM provided a lazar alignment tool to check and repair issues the all of the pulleys under the belt with a TSB to address all issues for pulley alignment.

Using the gator back belt may or will stop noise issue in the short term, but many shops tech have told me the noise issue problem came back and required other repair. Usually with a new idler or live tensioner pulley, common wear items.
Alternators and some A/C compressor's are in brackets that have sliding bracket inserts that are frozen and do not move them a component is replaced, allowing for misalignment when installed. As anything is possible, make sure these slider inserts are moveable and lubricated to allow the component to seat tp the rear of the bracket. While some require shims to adjust alignment, the tangentially mounted compressors do not have this issue.

If the new belt works for you, then hope it last, other wise the OEM belts would have lateral cuts like the gator back belt from the factory.

Re: Stumbling after start up
 

This was also my thinking on the subject. I will try the above. I think the alternator was recently replaced so that could be it. I also noticed a bit of runout on the balancer so that is sitting there waiting to be replaced. My idler doesn't look the best either. I just wonder why my 1994 SBC systems have never had this issue and I have two of those.

Re: Stumbling after start up
 

This morning, I started up and no stumbling, just the squeaky belt. Got two errands in and still running good.

Then on the third one it was back. Stumbling and lots of hesitation. I was stuck in a crowded parking lot so I gunned the engine in neutral and still would not clear up. It wasn't until I got on the road and really opened it up that the truck smoothed out.

Got home and tore into it again. Used MAF cleaner and Throttle Body cleaner. Checked two plugs and were fine. Removed belt and started, ran smooth. Spun the alternator and found the squeak. Everything else spun smooth.

Got a new altenator and installed. Smooth and quiet for now but definately on a wait and see status.