MAP is the manifold pressure. What are the units on your reading? Post cold engine KOEO and then idle and again at 1650rpm (Les and I did some benchmarks at this rpm).

George

 

George,
These are the readings from my Autrel hand held reader, MAP {KPA}
Cold engine, Ambient temperature 85 Degrees

Key on Engine off 97
Idle 625 RPM 34
1650 RPM 26

There you go,
Thanks,

Donald

 

I'll help translate

97 KPA = 28.64 in.Hg (barometric pressure per sensor)
34 KPA = 10.04 in.Hg of pressure (28.64-10.04=18.60 in.Hg of vacuum at idle)
26 KPA = 7.68 in.Hg of pressure = (28.64-7.68=20.96 in.Hg of vacuum at 1650 RPM)

 

Les Myer,
Thanks for the translation, but what does this mean to me and my situation? are these numbers normal, or off the grid? is there something wrong with the sensor or motor?
Thanks,
DB

 

I figured George would answer that. Don't make the mistake of following more than one person's advice at a time (why I didn't comment further). Myself I'm not seeing anything wrong with those readings, but it's not me that is helping you through this. Obviously I think that comparing sensor readings to a known good vehicle is a valid thing to do and a strategy that can be revealing, since that is what I originaly did with George. I suspect he will now ask you to check some other things that he has known good data on. It can be somewhat difficult if the units are not the same between scanning software, or if one scanner does more things. You might be asking more than can be delivered over the Internet, especially if you are unwilling to get the equipment/tools required. If your wife has an Android phone, you may want to borrow it and spend the $35 for the BAFX Bluetooth OBD2 adapter and Car Gauge Pro (or whatever app George prefers). How much is your time worth?

 

We may be on to something. Your MAP sensor reading is reasonable at KOEO because it reads essentially 14.2 psi which is atmospheric pressure at sea level - check. At idle and 1650 rpm however, your MAP sensor is seriously under reporting or your manifold pressure is much lower compared to the "reference" value that Les and I generated a while back. BTW, I don't remember noticing the differences between my numbers and Les' numbers at 1650 rpm on this sensor back then when my engine was running very rough. That issue aside, your numbers are massively lower than either of our numbers:

You Les Me
KOEO 14.1 -- --
idle 4.9 6.5 6.7
1650 3.8 11.3 7.7

We need your manifold vacuum at idle.

Check for kinks or leaks in the vacuum hose to the sensor

We need to check the 5v reference to the sensor

Im going to have to agree with Les that if you want to do your own truck diagnostics and repair you need to improve your scanning ability. I did not have android either so i bought a low cost used android tablet on eBay, a BT dongle on Amazon and the two software packages that Les evaluated and recommended. This is a bare minimum capability, I actually went two more levels up from there and just bought a used snap on scan tool that does all OEM codes now but thats overkill for most people here. I also have high voltage induction probes for my scope (which I had anyhow from designing and building tube amplifiers). I digress, I will do my best to test our way through this
with what you have.

Have you ever looked at exhaust back pressure? Not likely due to good performance but it can cause your problem and this sensor reading


George

 

ALso, in light of this Mike may be on to something wanting your load calc because a bad MAP sensor would also preturb this reading as would an exhaust restriction. We also need to do that VE calculation

 

George correct me if I am wrong, the MAP sensor it self doesn't have a out bound vacuum fitting except the orange plug where it enters the intake manifold. Again correct me if I am wrong. there is a vacuum pipe fitting on the manifold that runs to a vacuum hose over to a T fitting on the far drivers side, where it connects to the charcoal cannister?
I ask this, as The first thing I found when I bought this rig was a slight pulse in the idle, from 650 to 900, then I found a broken rubber vacuum hose that went from that T fitting to the intake manifold, I replaced that line and the idle surge stopped and became rock steady but then the mileage really dropped before I was getting 12 MPG and after replacing that Vacuum line , the air filter, and the PCV valve the mileage dropped to 8 MPG.
Once it cools off tonight I m going to clean the MAF and replace the air temp sensor and do some poking around at the MAP to see if it is tight. I don't know if I am into buying more tools and software of take it to a shop to be scanned if we can't solve this here, as the money will pile up either way
I will get back to you.
Thanks for all the help

 

My bad. I see that our MAP sensors do not have a seperate vacuum line connection, carry on with your plan and let us know what you find.

George

 

George,Did a little work before the heat got me. I( had some extra money from selling crap on Ebay so I went ahead and bought a AC/Delco MAP Sensor Pulling the old one out gave me some new data, The clip that that locks down to hold the MAP sensor into the manifold is missing. The old sensor was a sloppy fit and very easy to pull out. See enclosed photo. I found also that I do not have a flex tube from my Air Box throught the radiator shell, so it is pulling air from the engine compartment. Don't know if that makes a big difference, but will try to get one.

I also cleaned the MAF with MAF cleaner and put in a new AC/Delco ACT sensor, I have a new Coolant temp sensor that I will put in tomorrow.
On firing up I got my first code, it didn't last long then went away, a Random Cylinder Misfire code, don't know why, but it came on for a moment and it didn't get stored. I think it's from cleaning the MAF and the new ACT sensor
Once it cools off tonight I will run the all readings again, although I think I should wait till I get a clip for the MAP and an air hose from the dealership tomorrow