1999 s10 blazer 4.3 stalling

I've been working on a customers 99 s10 blazer, its came back a few times for different codes, but mostly the same symptom.
vehicle is mostly original with 130k. Nice southern vehicle (NC) no rust very clean for its age. everyone once in a while she has come in with a cel p0300. never thought to much about it, she never said it ran rough or anything, so someone had already tuned up on the vehicle. plugs, wires, so i decided to do a new distributor with cap/rotor because old cap screws were stripped. did a crank relearn and figured that would take care of it, run of the mill stuff, so i thought. this was back in march of 2016

last week she said it bucked and jerked while cruising down the road and come to a stop and stalled out. engine had 10 mins runtime, so heat soak wasn't in my mind. she said it wouldn't start back up and just spit and coughed until finally she cleared it out and was able to make it home. She waited till the next day and brought it in, ran perfectly to the shop, about 6 mins away.

luckily this time it had more than just the po300, so codes were p0300 P0108 p01106.

map sensor intermittent high voltage? never seen that before.

I a'm not the best driveability tech, I run a shop that does a little of everything, alignments, tires suspension inspections ect.... so u have to bare with me. I am busy for about 10hrs a day and don't have as much time as i would like to properly diagnosis a vehicle. so I use identifix a lot and i can usually get by. so between answering the phones getting other tech to work on time, keeping him busy talking to customers when they come in.

so i type in the symptoms on the identifix and read through a few pages, look at some quick data numbers on my autel everything seemed normal at idle in the shop, no time to test drive, customers wanting vehicle back that evening, I went with a crank shaft sensor in hopes of solving that p0300 that I thought was causing the stalling when it warmed up. Let the customers drive it over the weekend.

come in this morning, lady shows up in her husbands truck saying her blazer did it again. This time same codes, map p0108
p0300, p1106

so i knew this time i should look more at the map sensor. hooked up scan tool, and fuel psi gauge and drove the vehicle home, recorded some data, that did hit the p0108 and p1106 set parameter the map did go over 4.4 for more that 1 second. I did a few wot blasts on the way home, after i did two wot the vehicle started to buck and jerk, i pulled into a parking lot to look at my scan tool and the vehicle smoothed out and i checked fuel psi all was normal so I thought, took off from parking lot with more wot accel and it was fine, coming to next stop it wanted to cut out, bucking and jerking so i proceeded to "clear it out" to hopefully make it home. once home it idled fine and couldn't make it stall.

the cel did flash on the way home when it was bucking jerking so it set a p0300 again, also set p0101 and the same map codes.
4.6 volts on the map for 3 seconds was the longest i recorded. I know these vehicle use mafs quite a bit but the readings i got with it were about perfect I think the map being so high caused that code.

so is the map sensor sending to high of voltage or is it accurately reading a super low manifold vac from a clogged cat or something? should i try to run it without having it connected or rule it out? I don't have a back pressure tester. but the car only starts bucking a jerking after some wot heavy load time.

let me know your thoughts and suggestions for quick tests i should do so i won't have her come back again with same problem...

Thanks for all the help in advance!


Adam,
Adams Automotive LLC

What does oxygen sensor say is going great on with the fuel when it starts bucking is it not enough fuel or too much fuel . If the cat were plugged you would likely notice it having less power than it should ,those things should be able to spin tire when you accel around a corner if it won't do it you could suspect a cat . Taking a temperature reading of it is a good way to tell if you have a infrared temperature gun . I doubt it's the map sensor it would likely stall when coming to a stop everytime. Start with Graphing the upstream 02 sensor and snap shot it while it's acting up .

Hi..
I don’t know if you have a scope or not but if so I would back probe the crank sensor and see if its dropping out. Why I say this is all your codes are sensors that share 5 volt rev signal and my suspicions lead to a bad sensor thus the crank. I have seen crank sensors do this many times. You may have to let the truck run for a bit of time but I am sure it will die.

Good luck…

Doc n2mx

a flashing cel always almost is an ignition misfire. id suspect the coil or the ignition control module... how many misfires do you have? in history?

okay, have done some further testing and messing... after a busy work day doing normal repairs, Yesterday evening i decided just to catch the customer on some lacking normal maintenance items that were suspicions, they are letting me drive the vehicle home from the shop and do as I would on my own car. I changed the fuel filter because I couldn't find record of the last time it was done. checked air filter it was decent. also
cleaned the maf when some approved cleaner... yep brake clean! was hard to tell if the resistors had junk on them because of the screen on the front side. looked like they had a find powder over them. remember I did have a maf code at one point, but also had 3 other codes.
wasn't about to change all the sensor i had codes for. so just cleaned the maf for now to test.

also had read lots of hits on the crank position sensor, I had already changed that last week for the original compliant of stalling. but did read some suggestions on adding a shim, when i put it in originally the instructions said to look at the original sensor and look for contact on the end (i did not see any) so i figured no shims would be fine. on a whim decided to pull the new cps and look for contact, didn't really see anything scaring it, but put one of the thin shims that cam with the sensor. (luckily i didn't throw them out)

and last year had installed a new distributor w/ cap and rotor because the cap screws were stripped. and I know from previous experience always slot the hold down so you can adjust the distributor for 0 Degrees of cam retard on the scan tool. and i know i set that when it was installed but i rekon as the timing chain wears more and has more slop it was -7 degrees once the engine got good and warmed up. so I re adjusted that back to 0 when the engine was warmed up.

then comes the map sensor, p0108 code... just kept confusing me because I had never personally seen that code on a gm, only time I had ever changed on is on a intake job when u try to remove them they break, so I stupidly decided to pull this on out just to have a look to make sure the hole wasn't restricted or something weird... well guess what? it broke upon trying my hardest to be gentle removing the little booger.
so it had to replaced then....got a bwd (better china) map sensor

then after all this tinkering about an hours worth, cleared all the codes reset the fuel trims, (which were always spot on) and gave it hell around the block again. it never acted like it was going to stall and never chugged after a wot pull. but we did flash a cel for a bit, and then it stayed on. I know that flashing means there is a cat destroying missfire. so drove back to shop p0300... look at missfire data all misses were on #3.... yep the one behind the steering shaft! fuc*ing GM

got my stubby socket to remove the plug. read the plug just to see if it was ever changed, someone else did the plug change last time. it was ac delco proper spark plug. looked to be okay, had box of new ones on the shelf at work put one new one in on #3 only. just because i never like to put a used spark plug back in.


cleared the code and also decided to do a crank relearn, but also did it after I changed the cps. figured maybe the shim could affect it somehow,

anyways long story short been driving it home after work for two days and doing some errands, doing plenty of wot pulls onto the interstate from stops and hasn't missed a beat. no CEL no stumbles no dying good to go..

I wish i had more hard data for you guys I know this is crude but I am not a diagnostic tech, just have been watching Scanner Danners videos and this blazer pissed me off, not many places to turn, wanted to find some hard evidence before I flung parts on it... bad thing is I don't really know what fixed it. I figure its cleaning the maf knowing the symptoms. it just never thew a code for it until mid way through and without a scope its hard diagnose one properly. will have to get a scope box for my autel now!

also in history there was like a thousand of misfires on every cylinder... not just one bank, and it always seemed to have great power on wot, just once u held it at wot it would start to break up then cough and spit and once in a while die. so clogged converters wasn't the case...

Idk what else to say, hope this helps someone.

I am going to let the customers drive it over the weekend before I charge for anything. luckily only had to actually change one sensor and one plug. along with the crank sensor....

Thanks for the responses guys. will keep ya updated

I have a 99 Blazer with the 4.3 engine which I bought dirt cheap because the owner put a "rebuilt" engine in it and it wouldn't start. After trying everything he could think of, he got frustrated and gave up on the vehicle. It turned out to be clogged poppet valves on several of the injectors. I have seen this problem on many Vortec engines, especially the 4.3. They also frequently have problems with the fuel pressure regulator dumping raw fuel into the intake plenum. The solution to both of these problems is the same. They make an updated style of injectors that eliminates the poppet valves. This design uses the more conventional looking multi port injectors with the control wires at the end of each leg of the "spider". This assembly also contains the fuel pressure regulator. This modification also makes the system much more tolerant of low fuel pressure, which is often a problem with Vortec engines. The key to diagnosing my problem was my Picoscope. I put an inductive probe on the coil wire and it clearly showed a no fuel misfire on 3 or 4 cylinders which were all next to each other in the firing order. BTW the dtc i had was P0300 random misfire.