Codes P0236 and P0251

[spark3542]

97 Suburban K2500 6.5TD

History:
Lots of bizarre codes, to the point that I suspected the PCM.
Replaced grounds at the engine.
Found 5v ref voltage for Optic sensor to be about 5.94v.
Got a junkyard PCM, and took to dealer to have it programmed.
Measured IP 5v ref signals...they read 5.0v

"New" PCM threw P0236 Turbo boost sensor code
Found no vacuum coming from vacuum pump.
Removed wastegate controller, made homemade spring loaded wastegate actuator.

Vehicle ran OK for about 300 miles, sometimes throwing P0236, then resetting itself.

In addition to P0236, I also now get P0251, and with it there is a massive reduction in power (like it's in "safe" mode). It will not exceed 2k rpm.

IP Ref voltage at PCM now reads 5.5v.

I'm suspecting that the incorrect ref voltage is causing incorrect feedback from optic sensor, and this is sending the computer into safe mode.

Any words of wisdom?

Thanks

Mark in MA

[gmctd]

Very good possibility - continue with inspecting\cleaning the grounds and connections in the engine harness, incl the lower firewall area where the harness lies adjacent the aluminum ac-return tubing

[rfuntime]

Codes 236 and 251 are relevant to fuel starvation. However, my 96 was doing almost the same thing, and I found that the wiring harness going into the computer would make a bad connection and throw those codes. Wiggled it a bit, hasn't been a problem since. Paul

[gmctd]

0236 is turbo Boost system

0251 is IP cam system

Neither is for fuel starvation, but, together with the other codes, could be power or ground or connections, or all the above.

[rfuntime]

Gmctd is correct. Going back and looking at my notes, my codes were 251 and 370. Sorry for the misinformation.

[spark3542]

Thanks for the help. I'm very frustrated with this vehicle. I've been chasing grounds and wiring for a while. Found the two grounds that attach to the frame rail on the pass side to be rotted away, so I replaced them with some good 4 ga. wire. Then I found the overvoltage ref signals from the PCM, and decided it was probably a bad regulator on the PCM board. So now I'm back to square one, but I'm pretty sure it's not grounding issues, pretty sure it's not the PCM, pretty sure it's not the sensors (because I got such a variety of codes previous...including 370 and crank position sensor fault).

--- Caution...venting rant:!---
What I wanted was a kid hauler that I could reliably take on 200 mile trips, and possibly move to alternative fuels,... what I got was a vehicle that sits in my garage, waiting for me to have some spare time to crawl around and try and find wiring issues, taking it on 3 mile test drives, only to have it go to safe mode, where I have to limp home with my flashers on at 20mph. Park it in the garage, pissed off, and let it sit for another few days until I have time and patience to find another angle of attack to troubleshoot.
---Done venting, thank you for listening.

I've been a chevy guy for a long time, but this one sure is testing my loyalty!

Mark

[gmctd]

Turn the Ign on, but don't start the engine.

DVM black lead to the negative battery post -

DVM in DC volts, red lead to each ground point where a black wire comes out of the harness, following thru right into the cab and the grounds there, including the PCM grounds at the connectors - you're looking for any increase in circuit potential over 0 volts as you move DVM lead further away from the battery post.

PCM +5v reference may be correct if measured to the pcb, but not if measured externally thru connectors and\or ground(s) that are resistive.

And that +5v reference is the source power for many of the sensors, so you would get multiple failures from one bad connection or ground, resulted from oxidation, corrosion, dirty or loose.

[JohnC]

The most important ground as far as the PCM is concerned is at the rear of the passenger's side head on one of the intake manifold studs. I'd start there.

[spark3542]

I added a 14 gauge wire directly from intake manifold bolt to tan/white PCM ground wire at the PCM.

I drove about 60 miles on 4 separate trips without incident, then when I got on the throttle fairly hard pulling away from an intersection, sure enough, I get the all-too-familiar vibration, and then the check engine light and no power.

I pull the code, and it's P0251 only.

Now that I'm extremely confident that my PCM is grounded, what do the collective minds think my problem is?

Thanks

Mark in MA
97 Burb 2500 6.5TD

[Wink]

I have a 2003 Express in our Fleet with a intermittent p0251 [ optical sensor failure ] P0251 is the generic OBDII code that gets reported when the Stanadyne Injector pump can't read the optical sensor properly. VERY intermittently we get a Check engine light , then rough idel, then No start until it cools down. Already replaced the pump mounted driver a month ago which at that time i had no check engine light. To make long story short its going to the dealer for a New Injection Pump. I believe its under warranty until 120.000 miles. Sounds like you may need a IP pump also. Monitor you Cam sensor PID with a Graphing Scanner such as a MODIS and watch for missed cam sensor Signals. I believe when it hits 8 missed signals it turns the check engine light on and De rates to limp in mode. I thought a read somewhere that anything more then four 4 to 6 missed signals and its getting a new Injection pump. GOOD LUCK .

[gmctd]

The Optical Sensor may be failing, but dirty\cloudy fuel can cause that P0251 DTC - what fuel or additive are you using?

If you're less than 120kmi, the IP is still in warranty - do a search on this forum for a TSB to that effect.