This weekend was my first opportunity to tow with the truck. I tow a 22' 5'th wheel which weights 4400 lbs dry, loaded with our stuff it's probably pushing 6000 lbs. The trip this weekend was to a campground about 100 miles away. On the trip out on Friday the truck ran great and the SES light did not come on. On the way back on Sunday about 30 miles from home, I stopped at a truck stop to top up. The ramp getting back on the highway was the beginning of a long up hill grade and I had to step on it pretty good to get up the speed. Before I reached the top of the hill the engine icon came on. I thought at the time the truck might have felt a little lazy, however it was a long grade and I'm still not sure what is normal for this truck. Anyway it still seemed to be working fine all the way home. When I got home I read the following codes: P0101 (Mass or Volume circuit range performance problem), the truck had set this code last week while not towing, it went away by itself and I have not had a chance to pull the sensor to inspect it, however I will later today. The second code was P0236 (Turbo boost Sensor A circuit range/performance) and the last was P1406 (EGR Valve Pintle position circuit). Any help interpurting these codes would be appreciated. It looks like I might have multiple problems be maybe there is a comman cause. BTW I did not see any black smoke during any of this, and the mileage while pulling was 15 mpg (imperial Gallons), Thanks.

i have had the p0236 several times in the past when i run the turbomaster too tight and make too much boost. when mine throws this code it starts cuttin fuel at high boost rates. gettin an aftermarket map sensor would solve this code for my setup. i dont know about the others though.

Vacuum problems cause both EGR and wastegate problems. It's a starting point.

Check out this;

http://shell.deru.com/~sgn1/AW11/mod/images/turbine.jpg

What is that? It looks like he's made turbojets out of turbines.


Make sure there's 15" vac at the turbo and 20" vac at the baro sensor. A weak vacuum signal, broken lines, or a failing solenoid will cause power problems on an S, the first code is the MAF,iirc.

Thanks for the info QM, I work in a hospital and I have some guages around. I have a vaccum guage that reads in inches of mercury. I'm assuming that when you say to look for 15 inches at the turbo, it is 15 inches of mercury? I plan to unplug the vac line that runs to the turbo and measure the vac with this guage. Is the vac supposed to be fairly constant, or does it change with engine speed? Also, I'm not sure where the Baro sensor is,, could you or someone else let me know where it is? Thanks.

Inches of mercury, yes. The Baro is on the firewall above the brakemaster reservior, a

T in the vac line shares the vac between the egr and baro.

I made up my vac gauge and did some testing when I got home. First, I measured the vac coming out of the pump where it enters the 3 solenoids, it is a solid white tube. Vac measured there was a steady 25 in/Hg. Then I measured the vac at the turbo. I T'ed my guage and observed 11 in/Hg at idle, and it would increase to about 13" as I applied the gas, but it never really reached 15 inches. Next I noted that the line to the baro sensor and the line to the EGR valve are T'ed together, so I unplugged the line from the EGR and attached my gauge directly. At this point I got some strange readings. The vac would start at 0" and stay there for a few seconds, then it would build quickly to 8" - 10", then drop back to 4" or 0". It did this 2 or 3 times after startup, then settles in at 4" or 0", When it went to 0" I waited 2 minutes, but never saw vac again. If it settled at 4", it would also seem to stay there, I waited about 3 minutes once. I wasn't sure if this is were QM meant me to measure 20", so I reattached the EGR line, and pulled the thick line from the baro sensor. This time I also got 0" and no vac appeared. I stopped the engine and checked my connections, they looked OK so I started the engine again and reved it, this time when I got out and looked at the guage, I saw a steady 20 inches of vac on the guage. I stopped and started the engine several times at this point and about half of the time I saw 20", the other, I saw 0". Finally I reattached the hose to the baro senor, and this time I T'ed in at the senso. This time I either saw, 0" or 4", which is the same as the first test at the EGR valve. So it looks like I have a problem around the EGR, Baro sensor and soleniod. I get 0" or 4" when the Baro is in the circuit, and 0" or 20" then the Baro is out. I'm thinking I should probably replace the Baro sensor and the soleniod valve that the EGR and the Baro line are tied into. Any advice would be appreciated. BTW I took pictures of all my measurements, but I haven't figured out how to reduce them in size, so I did post them.

Can you snake a rubber tube into the cab from engine bay while driving, (route it so it does not get pinched off), revving and observing at idle isn't really representative how the system is supposed to work, I suspect the WG sol is laying down on the job more than I suspect the baro sensor, as the code you list is a turbo performance code not a EGR one, also could be a boost sensor bad, do you have a boost gage?

At 25" from vac pump you are getting a good vac supply reading, next I would tee into WG actuator line after solenoid while driving, PCM needs a driving load to meter fuel & boost in correct relationship, sitting at idle testing can skew your results, while driving you need to be seeing 15-20" vac at WG actuator, if you are getting that I'd be inclined to suspect the boost sensor, they can fail high or low your code could be giving false hi input (it happened to me), a scan tool that reads boost sensor input to PCM can help here to see if electronically we are getting high or low output from sensor, vs a boost gage reading if you have one of those.

Thanks Turbin Doc - The previous week, my truck threw a single MAS code which cleared by itself. I finally got around to removing the MAS sensor and have a look at it. All 3 resistors were covered with oily crud. I carefully cleaned them of with alcohol and a Q tip. I 'm thinking the oil must have come from my K&N air filter, when I got the truck I bought the cleaning/oil kit and probably went over board with the oil. Anyway after I cleaned the sensor and replaced it I measured the vac going to the WG soleniod and was surprised to find that at idle, my vac to the waste gate has increased from 11" to 16". It has been 3 days now, and while I haven't towed with it yet, I was able to give it a good highway run where I was able to go WOT several times on some hills. I may be my imagination, but I think the truck felts a little stronger now and so far no codes. I have a boast guage coming, but I may try to extend the vac guage into the cab as you suggest and see whats happening the next time I tow. As far as the EGR codes go, I have been trying to find some more info on what vac should be present there. At another site I was told that the EGR valve should be closed most of the time at idle (no vac) and should only open during periods of heavy load or while decelerating. This is sort of what I am seeing with my EGR valve. Do you agree that VAC to the EGR does come and go and that the EGR valve is usually closed? If so I think that my EGR system is probably working OK. Thanks again.

I'd suspect the wiring connectors for the middle solenoid. Are they the original connectors or has GM added new ones? The vacuum doesn't make it passed the middle solenoid if the ground is intermittent. 10" is weak, look for loose fittings and cracked lines.

EGR sounds like from your description that it is functioning correctly, still leaning toward WG sol it'sself or wiring to it as QM says.

Wait, your '97 has a MAF sensor?
I thought this was only '98-up LD apps, maybe I was wrong (sorry TD)
This setup is GARBAGE.
If you tow frequently, and are tired of the problems, get a HD ECM, new, used, aftermarket, doesn't matter, just as long as it ignores the MAF and EGR BS.

Wait, your '97 has a MAF sensor?
This setup is GARBAGE.
If you tow frequently, and are tired of the problems, get a HD ECM, new, used, aftermarket, doesn't matter, just as long as it ignores the MAF and EGR BS.

Oooh I'm tellin the tree police TDG thinks low smoke stuff is garbage:D :D

Yes find a find a F PCM for your truck model year preferably if you are 4x4 it needs to be from a 4x4 truck, this becomes your "offroad only PCM" or a diagnostic PCM so you can identify which emissions component isn't working; just don't forget after offroading or testing to reinstall the S PCM ;) ;) ;) or at least on emissions test day.

Thanks for the replies - TDG yes the truck is a 97 and it does have a Mas sensor. QM I have checked the soleniod connectors and spray cleaned them. I also pealed back the harness a bit and the wires to all 3 soleniod look good. Don't know if these connectors are original or not. As far as getting an "F" ECM do I have to get the same year or is there a year range that will work. My truck is 2 wheel drive will a 4 work in a 2? Also if the wrecker has it removed from the truck, is there a way to identify an F from an S ECM? And finally what should I expect to pay for one at the local wreckers? I spoke with a person about the emissions tests for diesels in Ontario. He stated that the only test is a visual inspection for smoke,, exhaust gases are not measured. If that is the case, running the F ECM all the time should not cause a problem. Final question, once the F ECM is installed what happens to the EGR system? I mean does it have to be blocked off in any way, or does the F ECM simply not activate it? Thanks.

BTW still no codes after cleaning the MAS last week, big test next weekend, 150 mile tow for the May24 weekend.

He's right, snap test is all they do for diesels up here, floor it, hold it to the boards and if it doesn't smoke you pass.

Try squirting a little WD40 into the BARO sensor on the firewall while the truck is idling.
If one of the solenoids that operates the EGR is sticking, this will help free them up for a while.