Erratic timing

[Road Trip]

Still going to grab a vacuum gauge tomorrow and learn how to use it...

After you satisfy your curiosity in the engine bay, try to rig up that
vacuum gauge so that you can drive around as you normally would
with the gauge in your normal field of view.

Back in the carburetor days I did this, and learned a *lot* about
how to better choose power valve springs, advance curves, etc.

Lots of valuable insight to be had - highly recommended. (!)

 

[PlayingWithTBI]

After you satisfy your curiosity in the engine bay, try to rig up that
vacuum gauge so that you can drive around as you normally would
with the gauge in your normal field of view.

Or, hook up your laptop/scanner and watch your MAP sensor then convert kPa to inHg. Here's a conversion chart I quickly downloaded, there are other more detailed ones on the web.

You must be registered for see images attach

 

[scott2093]

Or, hook up your laptop/scanner and watch your MAP sensor then convert kPa to inHg. Here's a conversion chart I quickly downloaded, there are other more detailed ones on the web.

You must be registered for see images attach

I was wondering about that..will do.....
. I did notice when I pulled the pcv hose from the throttle body, the truck wanted to fly away and didn't appear to miss a beat....
PCV still rattles..maybe a year old....all the info about them is confusing...me having an unknown roller cam and motor for that matter makes me wonder if buying stock y/m/model all the time goofs things up sometimes....
Pretty sure truck is running better though. Will know in a bit.....
So the scanner would be a step....Possibly a vacuum gauge would give information the scanner won't if still on the search for answers?.. These electrical parts are starting to disappoint... The new oil sender has a mind of it's own jumping around and I'm pretty confident it's not the oil pressure....
The old one, a GM as well, worked fine and was definitely more accurate with regards to scaring me with the oil pressure readings....

 

[95 Tahoe]

Check your map sensor. I had erratic timing going on. Then finally it wouldn't stay running. The map sensor had failed.

 

[scott2093]

Check your map sensor. I had erratic timing going on. Then finally it wouldn't stay running. The map sensor had failed.

tx...
I had my scanner hooked up while driving and the map readings seemed real stable in the respective conditions like idling..... My idle is pretty smooth now...Really think it was the loose coil plug wire.
Am going to decipher my notes to see what kid of vacuum was going on in different conditions. I did pick up a Hazard Fraught gauge as well...

 

[Schurkey]

hook up your laptop/scanner and watch your MAP sensor then convert kPa to inHg.

Sort of. Not exactly.

A vacuum gauge measures pressure DOWN from atmospheric pressure--which varies with altitude and weather, but for ease of mathematics and memory, I'd say equals 30 inches of mercury, or 1000 millibars.

A MAP sensor measures pressure UP from ZERO. Zero in this case is the inverse of atmospheric pressure.

IF (big IF) atmospheric pressure at your location really was 30 inches (check your local weather report) and the MAP sensor shows 20 inches, you've got 30 - 20 = 10 inches of vacuum. If the MAP sensor shows 12.36 inches of pressure, the vacuum gauge would show 30 - 12.36 = 17.64 inches of vacuum.

Here's a conversion chart I quickly downloaded, there are other more detailed ones on the web.

You must be registered for see images attach

I would expect the scan tool to be programmable to read in American units instead of Metric. But maybe I'm wrong.

At any rate, that's a more-useful-than-usual chart, in that it converts both the metric/American units AND the pressure/vacuum. 100 kPa of pressure = 0 inHg of vacuum, etc.

So the scanner would be a step....Possibly a vacuum gauge would give information the scanner won't if still on the search for answers?

No. There's nothing a vacuum gauge connected to manifold vacuum can tell you that the MAP sensor connected to manifold vacuum can't, if you process the MAP sensor signal into something recognizable.

A decent scan tool should be able to graph both crank position and MAP sensor signal, so that you can watch the MAP sensor signal in relation to each cylinder's intake stroke. A vacuum gauge won't react that quickly, AND you have no way to correlate it to the individual cylinders.

 

[scott2093]

A decent scan tool should be able to graph both crank position and MAP sensor signal, so that you can watch the MAP sensor signal in relation to each cylinder's intake stroke. A vacuum gauge won't react that quickly, AND you have no way to correlate it to the individual cylinders.

great...
It's weird because for the first time ever, the cashier at Hazard fraught said that the gauge had a restocking fee of 20% so, unless it's defective, I'd have to pay the fee..lol

I'm going to go over my notes..Still getting jumpiness it seems and I can see the map readings, oil pressure gauge, timing advance..all moving in concert with the jumpiness in the idle...Strange because it ran great all morning....

 

[95 Tahoe]

great...
It's weird because for the first time ever, the cashier at Hazard fraught said that the gauge had a restocking fee of 20% so, unless it's defective, I'd have to pay the fee..lol

I'm going to go over my notes..Still getting jumpiness it seems and I can see the map readings, oil pressure gauge, timing advance..all moving in concert with the jumpiness in the idle...Strange because it ran great all morning....

That's because everyone is renting tools and returning them. Mine would run great after the computer was reset. Once it started to learn the fuel trims and timing it started to run bad. I had an aftermarket cam. Chipping helped. Ran better for a while until the map sensor started throwing a low pressure code and dying. Replaced it and now it runs great with a slight lean miss at idle sometimes. I haven't checked the timing since to see if it was still moving around. Trying to fix that miss at idle on these trucks will drive you nuts.

I just realized this is a fresh rebuild. Another thing to check is valve lash. If the lash is too tight it will cause a miss and timing to bounce too. Also, checked for crossed plug wires.

 

[scott2093]

I just realized this is a fresh rebuild. Another thing to check is valve lash. If the lash is too tight it will cause a miss and timing to bounce too. Also, checked for crossed plug wires

ah...another reason not to hijack threads.....
op fixed theirs in December so I squatted here.....
My motor is far from new....
Appreciate the advice... Yes ..a slight miss has always a thorn in my side but this is a little more than I'm used to.....seems to have started since I replaced my oil pressure sender recently and put a mechanical gauge in.....
Went away once I snapped a loose coil plug wire back into the cap but seems it's returned for some reason.....
I must have goofed something along the way.....

Do want to play with lifters soon...Been itching to get back around a half turn but I've been spoiled by the lack of tapping at start up a full turn is giving me.....

 

[PlayingWithTBI]

the MAP sensor shows 20 inches, you've got 30 - 20 = 10 inches of vacuum. If the MAP sensor shows 12.36 inches of pressure, the vacuum gauge would show 30 - 12.36 = 17.64 inches of vacuum.

The MAP sensor shows kPa, you still have to convert it 1st. When you turn the KOEO the MAP sensor will read atmospheric and elevation pressure too. At sea level it'll read 100 kPa, at higher levels, it'll read accordingly lower. Then the computer adjusts the tables itself based on that.

the MAP sensor shows 20 inches, you've got 30 - 20 = 10 inches of vacuum. If the MAP sensor shows 12.36 inches of pressure, the vacuum gauge would show 30 - 12.36 = 17.64 inches of vacuum.

I don't understand what the big deal is about - all of the computer's tables based on pressure are in kPa so, we don't need to convert to inches of vacuum, we just look at the scanner and it does it for us.

I would expect the scan tool to be programmable to read in American units instead of Metric. But maybe I'm wrong.

Yeah most do but, we don't think in inches of vacuum, we think in kPa when tuning - don't really need to convert anything.

A decent scan tool should be able to graph both crank position and MAP sensor signal, so that you can watch the MAP sensor signal in relation to each cylinder's intake stroke. A vacuum gauge won't react that quickly, AND you have no way to correlate it to the individual cylinders.

Unfortunately, TBI engines don't have a Crank Position Sensor. IDK how you can extrapolate it.