Jump to content

Bad Stumble/miss/backfire 1995 5.7 Suburban

abracadabra

By abracadabra
in Ask A GM Technician

Recommended Posts

abracadabra Enthusiast

abracadabra

Posted
Posted

Background: 

1995 Chevy K1500 Suburban 

5.7 engine, 110,000 miles 

 

Problem: 

Couple weeks ago started with a stumble and miss (no backfire). Got worse as a few days went by, and would not accelerate well at all. 

No SES lights. 

 

Work performed: 

Replaced plugs, wires, cap, rotor, coil, PVC, air filter. All AC/Delco parts. 

Old plugs looked fine, but gap had increased to .045. 

New plugs gapped at .035. Miss/Stumble continued. 

Replaced fuel filter. Same problem. 

Checked fuel pressure, 11psi (non-vortec, it''s TBI so 11 is good.) 

Checked both fuel injectors, both spraying well. No drips when off. 

 

Read thru nearly 60+ pages on forums. 

Removed and inspected original IAC. IAC was fouled up, replaced IAC. 

 

With new IAC, now engine has an even more significant miss and backfires. Engine tends to idle high (approx. 1200 and very slowly comes down to about 900). To get the vehicle to move at all without stalling the engine I have to continually feather the throttle and I can get it to about 35-40mph max. Once the vehicle gets up to about 35, I can let the vehicle coast on idle. If I touch accelerator to get some more speed engine will miss and stumble and backfire if I push it too hard. 

 

The MAP appears ok. Output is 4.8 with engine off, and about 1.8 with engine running. Since I found that the engine "sometimes" runs better with the coolant temp sensor disconnected. Engine idles at about 1800 with it off, which is understandable since PCM thinks engine is cold and throws a SES light. I checked resistance of sensor and it appears correct vs. a table I found. The disconnected CTS I think is just masking the problem when the engine tries to idle at it''s normal ~500. I can throw car in gear and car will eventually "idle" at 1500rpm up to ~30mph. Once I start touching the accelerator though (either at a stop or while "cruising" at 1500rpm idle), the engine will stumble. 

 

I also thought thought about the TCS. 

2 things that tell me it''s not the TCS. 

 

1 - I checked the CTS resistance/temp to a table I had found, and it pretty much matches up. 

2 - If the CTS were the problem, it should run well when cold, since I''d assume that when the engine is cold and the sensor is "stuck" on cold, the engine should run properly. No? The engine runs just as bad cold as warm. 

 

I''m beginning to suspect a timing problem. Either chain is loose/jumped, or pickup coil? 

Do I need to disconnect the "set timing connector" to check the timing? Or is that disconnected only when fiddling with the distributor? 

 

Any ideas? Thanks!

ziptow Enthusiast

ziptow

Posted
Posted

Reminds me of the problems I had with my 88...it was the ECM. Do you have access to another computer to try? I have seen a few OBD-1 vehicles with bad ECM's..."don't rule it out"!.

 

You can change the MEMCAL inside...but if you can make a complete ECM/MEMCAL swap for a test..better for you. I keep extra ECM's..send me an e-mail with your ECM number..might be able to help with the ECM..but my MEMCALs are all 454/auto.

 

Ed

 

[email protected]

minibike132 Senior Enthusiast

minibike132

Posted
Posted

Yea "don't rule it out" it could be a PCM. But the reason you are here is to find out what it is right. Lets try and diagnose it instead of throwing parts at it. Pull any precat O2 sensors out and go for a ride and see what you get.

abracadabra Enthusiast

abracadabra

Posted
  • Author
Posted

ziptow, I'll keep you in mind with the ECM.  I don't have spare ECMs to try in it right now.

 

sparkstech, I'll pull the 02s and see what I get.

 

 

 

Thanks guys.

ziptow Enthusiast

ziptow

Posted
Posted
Yea "don't rule it out" it could be a PCM. But the reason you are here is to find out what it is right. Lets try and diagnose it instead of throwing parts at it. Pull any precat O2 sensors out and go for a ride and see what you get.

 

 

 

Just trying to help...no one said go out and buy a computer...but it is an item "often" over looked..that's why so many "remanufactured units" are sold. Just keep the thought in your head..ECM's do go bad. If you want see if the O2 sensor is bad..stream it's data..to determine if it is cross counting properly. The stream data will indicate problems with the sensor or problems in front of it.

Archived

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

Go to topic listing

  • Latest Articles

  • Posts

    • Chuck FB
      Joke Thread

      By Chuck FB · Posted

      A little girl said her prayers every night before bed and she asked G d to watch after each person she loved. One night she forgot to pray for her babysitter and her babysitter died. A few days later she forgot to pray for someone else and they died. The next night she forgot to pray for her dad. The next morning her dad was terrified. A bus almost crashed into him. He almost fell off a fifty foot ladder and almost wrecked on the way home. When he arrived home, he said I had a terrible day. His wife replied, “You think you had a bad day? My aerobics teacher died during class.
    • Jsdirt
      What did you do to your "other" car (aka musclecar) today

      By Jsdirt · Posted

      Nice! I had one here, once. The car dealer I used to haul for stopped by with it. That was over 10 years ago now. Those are SUPER rare these days, especially up here! Never even see them at car shows anymore.   https://postimg.cc/0rvcNZ1W
    • KARNUT
      What did you do to your "other" car (aka musclecar) today

      By KARNUT · Posted

      I remember watching one of the auctions on TV. It just happened that they had two little red express trucks. My brother bought one new in 1979, still has it. So I hung on to watch them cross. The retro redone example brought more bids and went for more money than the original one did. My brother has redone his 3 times and raced it. He’s now in the process of making more tame for cruising. 
    • Jsdirt
      What did you do to your "other" car (aka musclecar) today

      By Jsdirt · Posted

      I always upgraded the suspension and steering components - bushings and ball joints, and had older cars that I could out-corner ANY modern car with. I used to be able to take my '78 Cutlass on off-ramps at stupid speeds and make the sharp MA corners without even squealing the tires. So much fun. My '72 El Camino must've had work done by the previous owner, since that thing seemed to handle even better than the '78.   I think the suspension & steering on the '72 was much more advanced than the '64, so that probably just made a good system even better.   I'm the same way with vehicles. I hate having to worry about the paint on my '93 W250 Cummins. I'm not used to that, lol.   https://postimg.cc/mzKr55sY
    • Jsdirt
      What did you do to your "other" car (aka musclecar) today

      By Jsdirt · Posted

      BEAUTIFUL!! That a Cutlass F85? Or 442? My side-ID skills are lacking thanks to failing memory, lol. Love that color & year, too!   Yeah I was never big on entering my own stuff, but I always liked seeing what everyone had, the rare models, and what's been modified. That was always fun. These days I have barely enough time to do what needs to be done at home.    Oddly enough, here in MA there are more car shows than ever in history! You'd think up here it would be dead, but it's bigger than EVER!          Took the 940 for its maiden voyage with the carburetor. At first I couldn't get out of the yard! This originally had the "Bosch" system (our other 940 has Regina - single fuel pump in fuel tank, and space-age-looking ignition coil on driver's fender) - that has dual fuel pumps. One in the tank, and one under the center left of the vehicle. I had unplugged the external one thinking I wouldn't need it with the carburetor ... but the way I have the pressure regulator hooked up, 90% of the fuel volume is going back to the fuel tank, so cranked all the way down, the regulator is only supplying a hair above 3.5 PSI to the carb. Turns out the in-tank pump (15 PSI alone, instead of 45) couldn't handle the load. Plugged it back in and went for a rip. Ran great!   Throttle response is always amazing with carburetors. ZERO delay that no FI system can reproduce. Also fires up in half the crank rotations as before. Low end power was DRAMATICALLY improved. There was an ongoing lean issue with the FI that I got tired of figuring out, so this is a MASSIVE improvement! Above 50 MPH on hills it feels SUPER weak, though. This is more than likely due to the timing not being advanced enough by the computer, since it believes there are multiple failures. I can live with that through the winter. Will keep me from passing slowpokes where I shouldn't and getting speeding tickets at least. 😁

  • GM-Trucks.com Clubs

  • Popular Contributors

×
×
  • Create New...