TBI Stumble Diagnosis Help Request

[tony-baroni]

I have a symptom that I'd like to try and diagnose with TunerPro and hopefully someone on the forum can provide some help. My mechanic sat in the vehicle while it happened, but his only diagnosis was to hook it up to a scan tool, which shows everything is good.

The symptom, is what feels like an occasional miss under load. It's more frequent at altitude, though has started to happen at sea level. It is quite bad at altitude sometimes, to the point that at even freeway speeds, you can feel the van deaccellerate slighlty. I will be towing a large trailer over mountains this summer and I would like to fix this before hand.

I've discovered that it coincides with a brief rise in spark advance, by viewing the graphs in tuner pro. Here's an example.
Attachment 8880
Just after 8:36, it happens (a slight increase in spark advance, coincides w/ the stumble), again at 8:44, then 8:51.

I also found a couple bigger strange reactions in the spark advance, shown in these images.
Attachment 8881Attachment 8882

Any ideas on what could be causing this?
If anyone would like the saved full files from tuner pro, I can upload those as well.

Thanks for any help.

[1project2many]

Welcome.

It often helps others if you post information about the vehicle. Sometimes a problem is common to a vehicle or engine platform.

[tony-baroni]

Great suggestion 1p2m.

'95 Chevy 1 ton van w/ HD 350, OBD1 ECM. Engine was replaced before I purchased it, though from what i can tell it was replaced with a stock rebuild done by a good mechanic since all the systems I've checked are in place as the manual indicates they should be. All stock. I replaced the rotor and distributor cap, as they were showing significant wear. Wires and plugs looked good, so I did not replace those. My mechanic tells me drive it, don't sweat this small stuff.

One other item to note, I have identified #2 cylinder has low compression, in wet and dry tests. So it can be a leaking valve, not sure if it's exhaust or intake (I don't have a compressor). From my eval of the spark advance control, that should NOT affect spark advance, unless the fuel system is causing a knock/detonation. The IC system uses these inputs:crankshaft position, rpms, MAP, atmospheric pressure, engine coolant temp. I suppose an intake valve leak could cause a regular MAP fluctuation, but so far this stumble has not exhibited a regular frequency.

[tony-baroni]

bump

No feedback or ideas yet. If there are other variables I should be tracking, let me know and I will repeat the test to see if I can find other connections to this symptom to help in the diagnosis.

Thanks,

Great suggestion 1p2m.

'95 Chevy 1 ton van w/ HD 350, OBD1 ECM. Engine was replaced before I purchased it, though from what i can tell it was replaced with a stock rebuild done by a good mechanic since all the systems I've checked are in place as the manual indicates they should be. All stock. I replaced the rotor and distributor cap, as they were showing significant wear. Wires and plugs looked good, so I did not replace those. My mechanic tells me drive it, don't sweat this small stuff.

One other item to note, I have identified #2 cylinder has low compression, in wet and dry tests. So it can be a leaking valve, not sure if it's exhaust or intake (I don't have a compressor). From my eval of the spark advance control, that should NOT affect spark advance, unless the fuel system is causing a knock/detonation. The IC system uses these inputs:crankshaft position, rpms, MAP, atmospheric pressure, engine coolant temp. I suppose an intake valve leak could cause a regular MAP fluctuation, but so far this stumble has not exhibited a regular frequency.

[tony-baroni]

bump

Anyone help a n00b out?

bump

No feedback or ideas yet. If there are other variables I should be tracking, let me know and I will repeat the test to see if I can find other connections to this symptom to help in the diagnosis.

Thanks,

[jim_in_dorris]

If it is a low compression problem, it would be there all the time. I cannot say for sure what is going on, however I would suspect that the reluctor in your distributor is rusty and cracked. When that happens, it can get a misfire that jumps between cylinders. I have a similar problem with my truck, and the distributor is the only thing I have left to replace. I am not daily driving it right now, so I can work on it right now, that is probably what I will do next.

[tony-baroni]

Does anyone know how to data log misfires in tunerpro? I've explored the program as much as I can, and haven't found a misfire indicator/flag.

Also,I have seen the knock counts rise in fast fashion, but haven't figured out how to reset the # back to zero. Anyone know how that can be done?

[jim_in_dorris]

You can't really set knock counts back to zero, but it is a rotating counter, it will return to zero as it gets to its maximum and start over. The only time to worry about knock counts is while driving. Sometimes knock counts happen because of external noise, like the exhaust hitting the block, or some other external source. I don't believe you will be able to see misfires in OBD1, OBD2 has a misfire counter and DTC.

[tony-baroni]

thanks jim