Jump to content

Vehicle starts but quits when moved to lower altitude

distaff

By distaff
in Ask A GM Technician

Recommended Posts

distaff Enthusiast

distaff

Posted
Posted

Here is an odd one. I live near the Rockies (Calgary) at about 3,400 ft. I put a new GM 5.7 (L31) long block in my 99 1500 Suburban a couple of years ago, and it runs very nicely. Only real issue I've had, and it goes back a few years now, is that when I drive over to either the Seattle or Vancouver areas, and we are more or less at sea level, a couple of days after we get there, the truck will start and run for a few seconds and then quit. It will repeat this as often as I want to try it. If I leave it for about 20 minutes it starts and runs normally.

 

This has only happened after the truck has not been run for at least a few hours, and it has happened on about 6 different tips, it doesn't matter if it is rainy or sunny, and it always resumes reliable operation after less than 30 minutes - but it seems to need at least 20 minutes to recover. It never happens when we first get there, it is always a couple of days after we arrive.

 

Anyone heard of this in another vehicle?

Doug_Scott Senior Enthusiast

Doug_Scott

Posted
Posted

It really sounds like the ECM thinks you are back home. High altitude requires less fuel. At sea level, the vehicle will likely not idle on its own unless the mixture is fattened back up.

It used to be that the MAP sensor was what detects atmosphere pressure. Not sure what does it now, maybe MAF?

 

What may be happening is that when the truck does not start(starts and stalls), the engine is not cold. Waiting half hour will get the engine into a cold state. Fuel mixture is fattened up on a cold engine. That would overcome the ECM thinking you are at high altitude. It will likely take a monitor of some brand that can give you the sensors current state without the engine running. Avoid at all costs listening to someone telling you to "try changing xx part". Unless they are paying for it, or will take the part back if it does not fix the vehicle right off. BTW, that place does not exist. Even if it does not fix it, they will toss another part at it, while telling you the first part went bad, and took out the second part.

Duelin Enthusiast

Duelin

Posted
Posted

If it is a 99 model year then you should have a MAP sensor. Next time it happens disconnect the MAP sensor and see what happens. It should go into default mode. If that works then you could have a small vacuum leak.

distaff Enthusiast

distaff

Posted
  • Author
Posted

Doug - it has only happened with a cold engine.

 

Duelin, I'll try that.

 

I am really stumped because it usually happens a few days after I get to lower altitude. I recall being told that the adjustments for fuel mix are not real-time with this generation of ECM, and was wondering if it was happening when the ECM was trying to re calibrate.

Doug_Scott Senior Enthusiast

Doug_Scott

Posted
Posted

Doug - it has only happened with a cold engine.

 

Duelin, I'll try that.

 

I am really stumped because it usually happens a few days after I get to lower altitude. I recall being told that the adjustments for fuel mix are not real-time with this generation of ECM, and was wondering if it was happening when the ECM was trying to re calibrate.

 

If it is only when cold then I think you will find it is the MAP sensor or a vacuum leak. A cold engine requires a slightly richer mixture for the first minute or so. If the sensor thinks you are in the mountains, it leans it out to compensate for the low air density. A cold engine at sea level needs more fuel than one at high altitude.

It may be a vacuum leak that goes away as engine heats up. Look around intake ports between manifold and cylinder head. Not sure they use these days to find vacuum leaks, before o2 sensors we used either acetylene or propane. Do NOT use oxygen.

distaff Enthusiast

distaff

Posted
  • Author
Posted

Doug, that is an interesting insight. I was told I had a small vacuum leak that the mechanic couldn't find and he ran propane over the intake. May have to charge it with a smoke machine and look for a wisp.

Doug_Scott Senior Enthusiast

Doug_Scott

Posted
Posted

Doug, that is an interesting insight. I was told I had a small vacuum leak that the mechanic couldn't find and he ran propane over the intake. May have to charge it with a smoke machine and look for a wisp.

 

I can't see how a smoke machine would show a vacuum leak. Unless you are looking to see if the smoke is being drawn towards the vacuum leak. I would think that would be tough to see given the amount of natural air movement around a running engine.

Doug_Scott Senior Enthusiast

Doug_Scott

Posted
Posted

You have to pressurize the intake and then look for a wisp of smoke.

 

That has to be a fairly intensive procedure. You have to make sure all valves are closed, all vacuum lines are plugged(and doing that alone may fix the issue, until you put back after finishing the test), and you cannot count on where the smoke is actually coming from.

If you have the system sealed up to test this, why not just pressurize the system and see if it holds pressure first, then apply a vacuum to the system and see if it can hold a vacuum.

Archived

This topic is now archived and is closed to further replies.

Go to topic listing

  • Latest Articles

  • Posts

    • fondupot
      Exhaust heat while towing!

      By fondupot · Posted

      You can install tailpipes that direct the exhaust sideways vs the OEM one that goes straight back. There are several on the market, do you own research. HOWEVER, you need to make sure the Venturi vents are still intact, otherwise it will throw a engine code. There is a sensor somewhere that detects back pressure on the exhaust. And if it doesn't collect the correct metrics, it will throw the truck into limp mode. 
    • Atlas
      Wow I'm in a pickle. (Broke down truck) and a story.

      By Atlas · Posted

      "Teardown" is complete. Head surfaces are clean and ready, new intake is prepped and ready. Coolant drained and water pump is removed, radiator is removed, new hoses and pump are ready.   The replacement intake is also from a '95 (with I think 260k, the odometer said). Noticeably less pitting where it covers the rear coolant passages. I'm not sure if that's a tribute to more frequent use, more frequent coolant changes/upkeep, or if the intake was machined at some point. But it appears I found a good one.   My coolant hoses were original, and the lower hose still retained the factory spring inside. It's not really rusty, but it's got deposits all over it. Unsure if I should re-install it (will thoroughly clean it first). Someone said hose technology has gotten better since 1995 (has it?) and I don't need the spring. I think modern EPDM is what made those hoses last from 1995-2026, so it's a question of how much suction can the water pump really pull, and will it collapse the hose without a spring?   I'm stopped and waiting for parts, however.   When I first brought the truck home, the fan was intermittently hitting the radiator shroud. I could see someone had monkeyed with the shroud (put a bunch of cardboard as a spacer between the upper/lower shroud halves. Someone said the motor mounts have likely failed and the engine has sagged. I wasn't sure I believed that. The engine is really solid in its mounts: To remove the old intake I *just* nearly permanently welded to the heads with freshly cured RTV and gasket snot two weeks ago, I literally had to use the arms on my lift and a chain with a bolt attached to the intake to pull it loose. It almost picked the front of the truck off the ground before ....scuuuuuuuuuuup.... I could actually hear the gaskets tearing and it broke loose and the whole truck settled back down. Motor barely moved.   SO, those parts I'm waiting on... The lower radiator mounts/bushings had completely failed. The radiator needs to be insulated from the rad support, and the bushings help the rad stand at the correct height. The plastic "baffle" was also destroyed at some point. Someone had attached the shroud to it incorrectly, overlapping a joint that should have been underlapped, creating another suspect issue potentially causing things to hit the fan. Broken tabs, side is busted off, and it's perma-formed/bent where it was jimmied incorrectly.   The baffle? Unobtainum. Long since discontinued from GM, of course. Aftermarket/capa/chinese? Also unobtanium. In GM's infinite wisdom, it appears there are at least two different variations whether the Blazer is 2 or 4 door, and I can't yet tell if they shared the same part with the S-pickups. Plan B, if I can't find a viable used one, will be to create a bracket to re-assemble the critical joint I need to seal a gap with the shroud. But that will bug me, it won't be...perfect.   The water pump bolts appear to have been installed with...plumber's putty. I guess, sure, if it works...? I've found a few "backyard fixes" like that. Someone's been here before....   If I can't find someone to jam bamboo under my fingernails to fill my free time this week I think I'll install the driver's power window motor and replace the rear door speaker that's inop. I've got a genuine GM "9.5" ohm working replacement. Why GM used a non-standard resistance....nevermind. Of course they did, you can get more sound from a cheap factory amp that way.   And the saga goes on...
    • glenn27
      Different requirements for oil filters?

      By glenn27 · Posted

      // Thank you, also for your  response. More information is always good. This 2013 GMC is most likely my 'forever truck"-I'm 77 and retired...and truck only has 42K on it-very clean-so I try to keep it that way.  Tks again for your  response- glenn
    • smooth68
      Alternate wiring for Kicker sub with Bose system. (DO THIS)

      By smooth68 · Posted

      I started my install this weekend. Everything was pretty straight forward but I ran into one issue with running power through the firewall. The nipple grommet on my High Country already has a small harness running through it - has anyone else ran into this issue? And on the engine bay side, does the wire basically just penetrate the factory wiring boot? I did this on my '22 Trail Boss and it seems much easier but thats been a few years. My other thought would be possibly drilling a new hole adding a rubber grommet just to the lower left of the factory hole. Thoughts?   Has anyone used a T-harness off the Bose amp and obtained power from there? It seems a little odd that such a small amp needs 3 power leads
    • glenn27
      Different requirements for oil filters?

      By glenn27 · Posted

      ///// Ok--Thanks for that info--I could not find yesterday when I looked for the specs on that NAPA filter what I wanted. Their website is not very user friendly. I buy all teh filters for  our three vehicles once or twice a year. Have always run NAPA Gold on everything I own....I buy it thru my Son's account for his business-which he has a Heavy Equipment repair shop-one reason he uses NAPA so much-they have such a full line of coverage for just about anything/anywhere- He gets s  good discount because of volume bought. Anyway-Thank you -Sir for your response.   glenn

  • GM-Trucks.com Clubs

  • Popular Contributors

×
×
  • Create New...