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Bad Stumble/miss/backfire 1995 5.7 Suburban

abracadabra

By abracadabra
in Ask A GM Technician

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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.

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