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Bad Stumble/Miss/Backfire 1995 5.7 Suburban


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QBert's Avatar
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August 26th, 2008, 2:50 PM   #1  
Bad Stumble/Miss/Backfire 1995 5.7 Suburban

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. Is PCM supposed to advance timing upon acceleration? Does this thing have a pickup coil in the distributor? Is it testable?

Any ideas? Thanks!

 
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August 26th, 2008, 4:17 PM   #2  
RE: Bad Stumble/Miss/Backfire 1995 5.7 Suburban

I had the exact problem and did the same work, to the letter almost. The is a temperature sensor next to the thermostat housing on the intake manifold was the problem. This is the sensor that tells the computer what the coolant temp. is. Ifthe pigtail is loose or corroded, like minewas,and there is no clearclean connection the motor will think the sensor is saying its real cold, making the motor run real rich making acceleration impossible in fact flooding the motor out like a stuck choke on a carb. I could not drive the truck, by the way it is '95 suburban, till I changed the corroded pigtail and sensor. Look down into the plug and outlet for brown rust and clean it out w/ contact cleaner and tiny file. Unplugging the sensor causes the same problem w/ no input to the computer. GOOD LUCK.

 
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August 26th, 2008, 4:18 PM   #3  
RE: Bad Stumble/Miss/Backfire 1995 5.7 Suburban

My 05 25004x4 454 had a similar stumble as you described.
I found the coolant sensor went bad and was always telling the pcm the engine was cold and therefore the pcm richened the fuel, once the converter became clogged with soot it started to stumble and would not go when you accelerate.
I replaced the coolant sensor and the converter and all was back to normal.

I would suggest just unhooking your converter and see how it drives.

Jack

 
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August 26th, 2008, 5:19 PM   #4  
RE: Bad Stumble/Miss/Backfire 1995 5.7 Suburban

I also thought thought about the CTS.
2 things that tell me it's not the CTS.

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 fidlling with the distributor?

 
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September 4th, 2008, 6:46 PM   #5  
RE: Bad Stumble/Miss/Backfire 1995 5.7 Suburban

I had the same problems and changed the entire emissions components (plugsto 'cats', I mean ALL). The problem was my fuel pump. Although the gauge read ok at idle, during exceleration it did not increase pressure. I changed the pump and now I have a new truck (lol). Very expensive though.

 
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September 5th, 2008, 9:42 AM   #6  
RE: Bad Stumble/Miss/Backfire 1995 5.7 Suburban

Fuel pump has been checked at idle and under load. As this is a pre-vortec engine, it doesn't need the ~60psi that the vortecs need. I've been told to check cat as it may be causing some backpressure. I'll check it out in the next couple of days when I'm a little less busy.

 
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August 11th, 2009, 12:55 PM   #7  
Did you find the problem? My92 k1500 has a similar problem ,idle surges until warm and the goes into limp mode and runs terrible. I replace the temp sensor same problem. If I jump the connector unplugged with a 700 ohm resistor and the engine warms up the surging at idle stops and will run good in closed loop even though it is probably running alittle rich. I believe the ecm thinks the engine is overheated and goes into limp mode when up to temp and the resistance on the sensor is around 250 ohms for 190f temp.

 
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August 11th, 2009, 9:02 PM   #8  
Do you have a vacuum gauge? I can give you some tests to perform to check for a internal engine problem ( t- chain, valves) and for a exhaust restriction

 
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August 12th, 2009, 2:32 PM   #9  
Yes I have a vacuum gauge. Please give me instructions for internal tests and exhaust test.

 
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August 13th, 2009, 10:04 PM   #10  
Connect the vac gauge to manifold vac. The brake booster hose is usually the best spot to connect. Start the engine the reading should be 18-21 and steady. If the needle is bouncing with the miss then the problem is internal in the engine, valve lifter,worn cam,etc..If the reading is low then you most likely have a ignition or valve timing problem. Next rev the engine up quickly to around 3500 and hold it. Vac should drop down quickly then come right back up to around where it was. If it goes down and stays down or comes back up really slowly you most likely have a exhaust restriction

 
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