Running rough and low on power

Shadowhunter

Newbie
Joined
Dec 10, 2023
Messages
1
Points
1
Make
Chevrolet
Model
Suburban 2500 4X4
Year
1995
Miles
192000
Engine
5.7L 350CI
Problem started with Running rough and low on power, no CEL. So far I found that the screen (for a lack of knowing name of part) on the MAF sensor was bent up pretty bad and sideways in the body. I took the screen out and left it out, cleaned the sensor, no change still running rough. I replaced Rotor, Cap, Plugs and Wires, (which weren't that old) no change still running ruff. That said I don’t think it needs a new engine as yet and I performed a compression test on all cylinders and they are all within spec. The top end Valve job was done at about 130K due to some blow by. I was talking to a diesel mechanic at work and I was then told that it might be the injectors so I replaced all injectors and did the Central Sequential Fuel Injection to Multi Port Fuel Injection (CSFI to MPFI) conversion upgrade. Still no change and running rough. I don’t mind doing the process of elimination parts replacements but I'm not sure what I should do next. I have read that the distributor is mediocre so im leaning towards that. I have also checked the pressure on the exhaust pipe thinking it may be the catalytic converter and it seems ok. Today it did something different though, I started it just to move it which I have done multiple time during the last few weeks and it backfired first and did not start until the second try when it raced up to about 4k RPM and then would not stay running, I tried starting it another time and it did the same thing raced to 4K RPM and would not stay running. I knew at some point I would be putting money into it and I have taken very good care of this vehicle since day one. It still looks real good and they don’t make 3/4 ton Suburban's anymore (couldn’t afford one anyway) and I plan to keep it much much longer.
So I'm looking for ideas as to what to do next and any help on this would be greatly appreciated. Thanks in advance. I am glad this forum is available and have used it before on my daughters cars.
 

billr

wrench
Staff member
Joined
Mar 12, 2007
Messages
7,869
Points
63
Well, we have a lot to talk about here...

1) I am wondering how a "filter" screen in the MAF sensor could have been bent? Maybe post pictures of the MAF sensor and any damage that is still apparent. My perception is that MAF sensors are fairly delicate, so any event that could damage that screen may have also damaged the sensor element. More about this at the end of this list.

2) I would like to know specific compression numbers, and how the test was done: how many strokes? all plugs out? throttle wide-open? cranking speed and battery voltage normal? altitude at your location? tested both wet and dry?

3) You mention valve work to correct blow-by, but "blow-by" commonly refers to leakage of combustion gases past the piston rings into the crankcase; no relation to valve condition. Can you elaborate on the work done and symptoms that led up to it?

4) Is this a traditional Chevy dizzy (distributor), mounted on the rear of the intake manifold? I have always found those to be rugged and reliable. What have you read that makes you question condition of the dizzy?

5) What is exhaust back-pressure, and was it taken with the engine loaded and at what rpm?

6) That speed "flaring" to 4000 rpm is a strong hint of a some kind of vacuum leak. A simple vacuum gauge may give some good clues Also, disconnect the vacuum lines to the power-brake and PCV; plug/cap the ports right at the intake manifold. No joy? Do the same with any smaller vacuum hoses, such as those going to EVAP system. Block off the EGR ports on the intake manifold, eliminate any chance of problems from that system. Once the engine fires and starts racing, slowly slide a block of wood over the opening to the throttle-body. If that kills the engine, then we can suspect the IAC system. But, if blocking the TB opening does not kill the engine, then we know there must be a vacuum leak somewhere.

7) Lastly, if you are intent on keeping this vehicle for a long time, you must have a way to view live-data. Use that to check MAF reading along with much else. This is an OBD1 system (or maybe "OBD1.5", since it is '95), but that 5.7L Chevy V8 was so universally used that there are many options for reading live-data from it!
 

grcauto

wrench
Joined
May 29, 2014
Messages
1,423
Points
48
I'd be looking at intake leaks.
With these GM systems you can unplug the MAF and they will run ok. Tap on them with engine at idle and if it causes it to stumble or stall the sensor is bad and needs to be replaced.
 
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