MAP issues (P0108)

[mudpie]

I searched...found threads that just died (I hate that...post what you found), and threads by people who were chasing multipe codes. I've got one code.
So my truck ('96, 5.7) was running fine, then suddenly started popping and backfiring a bit, and generally losing power. An hour later it died on me. It would idle, but if I give it any throttle at all it would die. I towed it home with a strap, and let it idle so I'd have brakes. On the way home the idle was surging from about 900 to 1500, then back to 900. Did that all the way home.

My brother has an ancient code reader, but it's an ancient truck, and I was able to get a P0108 code. I replaced the MAP sensor with a spare I had laying around, but I don't know what it came from or if it is in fact good. I seem to recall years ago having some sensors in the glovebox that I swapped in to see if they were good, and kept the good ones. I don't recall if this MAP sensor was one of those.

Anyway, when I put the spare in I noticed my idle has stabilized, and if I apply throttle very slowly it will rev. If I give it any kind of substantial throttle though, rpm's drop to a couple hundred and it runs like crap. May be purely coincidental, I don't know.

A new sensor is about $65 in my area, or I can order one online for $10 and wait a week. I don't want to wait. I need the truck. I hate to spend the money without knowing for sure though. Is there anything else I should check before I get the new sensor? I was thinking maybe a vacuum leak, and I sprayed carb cleaner all around the intake, but it made no difference.

Also, can somebody tell me what the numbers on the sensor mean? The one on the left is the one that came out of the truck and the other is the spare I had laying around. The part number seems to be the same. Is the number in the middle relevant to anything?
Thanks

 

[L31MaxExpress]

I do not have a definative answer but most likely a lot number that coincides with a specific run of sensors.

 

[east302]

Do you have a spare throttle position sensor or a scanner that can read its output (percent open) versus throttle position? Wouldn’t hurt to check the cleanliness of the throttle body.

The factory service manual has the diagnostic procedures for each code. They can be downloaded here…

88-98 Service Manuals

Over the years I have collected a bunch of GM service literature. Links to PDF Scans of service and STG manuals and a links to the Parts manuals. You'll need the FREE Adobe Acrobat reader to open these files. Get it directly from Adobe here {http://get.adobe.com/reader/} If you're not...

www.gmt400.com

 

[454cid]

Use that ancient scanner to look at live data. Also, it would be a good idea to watch an educational video on how the sensors all play together, if you don't know it all by heart. Schrodinger's Box is a good one.

I've got no MAP experience, but I have had issues with the other sensors.... TPS, MAF, O2.

 

[Schurkey]

So my truck ('96, 5.7) was running fine, then suddenly started popping and backfiring a bit, and generally losing power. An hour later it died on me. It would idle, but if I give it any throttle at all it would die.

What is the fuel pressure? How old is the fuel filter?

Any chance the distributor gear is worn? What does the scan tool show for camshaft sensor/crankshaft sensor synchronization (Cam offset).

How old are the cap, rotor, plug wires, plugs?

 

[Pinger]

Wrote the below yesterday but for some reason didn't post it. But as some of it is being hinted at by Schurkey, here's what I was thinking yesterday:

I'd check all the sensors and actuators are getting their 5 or 12V feeds and grounds and their resistances (where possible/appropriate eg, TPS) with a multi meter as a start.

I had the surging idle thing a while back (after changing the inlet manifold gaskets) along with reluctance to start and take with throttle when it did. I checked all the sensor/actuator stuff as above (fuse ENG-1 was missing - so no MAF sensor functioning until new fuse fitted) and took the IAC valve out, cleaned it and refitted it (without moving its position) and everything came good. Why exactly I don't actually know. I did have a bit of jiggling with the (tamperproof) throttle stop screw which had been tampered with.

Popping/backfiring implies slow burning weak mixture, ignition timing awry, or cross-firing on HT side of things. Bad sensor data could deny it fuel. Physical check of HT leads and dizzy cap/rotor might identify problem there. If the dizzy body rotates on an L31 it can cross-fire apparently.
I'd do the above physical checks and if nothing obvious found, probably time to hook it up to a scanner to read live data.

 

[mudpie]

Messing with the truck some more and I briefly got a code for the TPS (P0122) along with the MAP sensor (P0108). I cleared the codes and only the MAP sensor code has come back.

Do you have a spare throttle position sensor or a scanner that can read its output (percent open) versus throttle position? Wouldn’t hurt to check the cleanliness of the throttle body.

Thanks to a liberal return policy at Pick-n-Pull, I now have a collection of 4 MAP sensors and 3 TP sensors. I've swapped every combination of them and it's made no difference. Obviously they're junkyard parts, and I got them just for the purpose of determining what's wrong without throwing $65 sensors at the truck randomly, but I find it hard to believe they're all bad. This leads me to believe my issue is not either of those actual sensors. I've cleaned the MAF sensor with the proper cleaner, and cleaned the throttle body with carb cleaner. No change.

Use that ancient scanner to look at live data. Also, it would be a good idea to watch an educational video on how the sensors all play together, if you don't know it all by heart. Schrodinger's Box is a good one.

"Live data" on something that's probably as old as my truck is kind of a funny concept. It gives me my average fuel economy, my coolant temp, IA (intake air?) temp, and codes, all of which I understand. It also tells me that at idle, my TPS is zero, and at 1200 rpm my TPS is 2, whatever that means. Also, my LOD is 5, my PID04 is D, and my PID06 is 82. All very enlightening stuff. Pics below.

Schrodinger's Box looks to have some very good content, and I do appreciate that suggestion. I must admit though that it's not high on my priority list at the moment to spend an hour watching a 2 part video on all the intricacies of how a throttle position sensor works. I probably should, and probably will, but right now...? Only so many hours in the day.

I'd check all the sensors and actuators are getting their 5 or 12V feeds and grounds and their resistances (where possible/appropriate eg, TPS) with a multi meter as a start.

Physical check of HT leads and dizzy cap/rotor might identify problem there.
....if nothing found, probably time to hook it up to a scanner to read live data.

I'll check those things.
This kind of stuff is frustrating, given that the whole point of on board diagnostics was to make our lives easier and point to the problem. Instead the computer is saying "Hey, this sensor is bad. No, just kidding....you figure it out". I love this truck 99% of the time. The one percent raises my blood pressure and/or my blood alcohol level.

Unfortunately, hooking it up to a scanner involves a towing bill to a shop, who will then hit me with a bill designed to help pay for their $2500 scanner. All that for one MAP code.

 

[east302]

For a 98, the EGR, TPS and MAP share the same reference circuit from the VCM and that TPS code is a low signal voltage code. Not sure if it’s the same for 96 or not.

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Checking for damage on each wire from those two sensors to the VCM wouldn’t hurt. The manual does have the step-by-step diagnostics for each code, so that would be the ideal way to resolve those codes. I’m not sure if a 96 is posted in that link to the manuals, but using the next-latest year would be an option. Otherwise, alldatadiy has the same information for each year but it’s a subscription site - around $70/year I think.

 

[mudpie]

I checked the plugs and they're black. Wires look good, but I found the distributor cap very loose. Apparently the screws that hold it on have stripped the holes in the distributor, so I couldn't tighten them. I seem to recall reading something about this being a common problem, and oversize bolts being available that cut new threads when you screw them in. I'll see if I can track down a couple of those tomorrow.

At this point I'm assuming the loose cap caused a weak spark, the plugs fouled, and ultimately it wouldn't fire under load even though it seems to idle ok. I'm hoping this will solve my problem, but if it does I'll be perplexed as to why I got the codes I got. The P0108 and P0122 codes are strangely specific for what is basically a random misfire.

We'll see what happens...

 

[454cid]

I'm not sure about oversize screws, but there is a metal bracket that fixes the problem, but if I recall it's expensive for what it is. I dont remember who makes it for sure, but I want to says its Dorman.