Tach not working, Service stabilitrack, rough idle!!!

king_strut · December 13, 2020

Hey Guy's, I am new to this forum and need some help pretty bad. I have a 2008 Chevy Suburban LT 4wd 5.3 flex. Here is the breakdown of whats happening. When starting the truck it takes a long time to finally fire up, when it does it is hesitating and shuttering a bit. While driving it has lower than usual power on acceleration even though it will still run 70mph and the cruise still works telling me it probably isn't TPS related. I have changed the MAF to no avail. In park there is no engine noise or valve rattle whatsoever but under a load it does rattle. ??? I am stumped and would love some input and insight from anyone with similar issues that has found a solution to this problem. I ran the codes and got no cylinder misfire codes but did get a crank position sensor code and an abs wheel sensor code. I look forward to any help I can get and would really appreciate it.

CamGTP · December 14, 2020

The crank sensor code and or fuel pressure could be an issue here.

Check fuel pressure for the long start cause. It's possible it could be low on fuel pressure when under heavy load as well. Also check fuel trims with a scan tool. If they are extremely positive that is either a vacuum leak or fuel pressure problem.

The crank sensor could seems like it could be linked to your clusters tach not working and the rough running. The service stabilitrak could go along with a fault like that. You're going to need a good scan tool with live data for that. Or just shotgun a new OEM crank sensor in it after checking over all the wiring.

Edited December 14, 2020 by CamGTP

king_strut · December 16, 2020

I changed the CKP ran the snap-on solus for DTC's and only got the P0335. I cleaned the wheel speed sensors before doing this so that cured the abs issue I believe. I have great fuel pressure and the scan tool is showing no engine rpms as well. It shows CKP rpms and CMP rpms bot no active counts on the CKP? I dont even know how that is possible. I changed the cluster and still no tach to no surprise since the solus showed no rpm data as well. I cannot do a relearn without rpms as the solus will not allow it. I checked the connector on the harness and stripped back the harness cover and inspected the wires a bit further back but saw nothing out of the ordinary there. I would like to know what wires at the ecm are the wires coming from the ckp so I could jumper them to see if maybe there is a break in the wires going to and from the ckp. Thanks.

riverbanks · December 16, 2020

AutoZone site,gmupfitter or 2008 Silverado pcm pinouts.pdf

king_strut · December 16, 2020

UPDATE I got the pin schematic and have 5.03v on my reference wire constant, I have 2.38v on my signal wire constant even though I shouldn't. I thought maybe a short in the harness under the engine so I cut in a new wire for my signal from the connector at the CKP all the way to the ECM. still getting 2.38v? I have no idea why? When I removed the wire from the ecm and tested I get nothing at all on it. I am stumped here.

king_strut · December 17, 2020

Today I replaced the cmp and pulled the entire wiring harness off and inspected it thoroughly but found nothing wrong with it. I checked voltage at the cmp and got 5.03v at the reference and 5.03 at the signal. low reference is 0v. still getting 2.38v at the signal wire on the ckp and I changed the ecm and bcm and still get the same numbers. Anyone got any ideas I am at a loss here! Thanks

riverbanks · December 17, 2020

Yikes,try gmtnation.com/How to Diagnose a Failed pcm,I tried to upload,no luck,there is some ohm testing on there,might be of help,I'll keep looking

king_strut · December 19, 2020

I have checked both the sensor which is perfectly fine and the reluctor wheel which is fine also. No wobble of flex at all. I get 4.7v with KOEO and 2.3v with it running on the signal wire for the ckp. I get the same thing on the cmp wires but the cmp signal jumps around from 2.2v-2,6v in that neighborhood. I get no pulse from the signal on the ckp at all just a steady 2.3v.

riverbanks · December 19, 2020

?,time to eliminate any non-essential fuse and relay,or modules,one by one,great good man,a wiper motor assembly can take it down,it all depends on how a module failed

king_strut · January 12, 2021

Well I have exhausted all my brain power and still haven't found the issue.If anyone out there has had these same or really similar issues please help me out here. This is driving me nuts!

GM_Cruze · July 6, 2022

@king_strut

Bump! Did you ever figure out what was wrong?? I have a similar issue.

christopher breed · July 8, 2022

'08 Tahoe ..Same... started 3 days ago. slow turning over when leaving work. Drove fine home. Started the next day and barely turned over ran very rough. Fuel fumes, no RPM's on the tach, service stabilitrack, VERY rough shifting into 1st gear. Parked for a few days and scheduled to take it in. Jumped it with my new NOCO jump battery (very cool). It ran perfect to the mechanics. Perfect. Shifting fine, RPM's on the tach. Ran like nothing had happened. Left it at the mechanics and called to get the scoop. Needs new battery and needs software update? $650. Sounded like alot to me. Passed on the repair. Ran rough all the way home. Can a bad battery cause all of this? the pcm? What can make it suck one day, run prefect the next day and then suck again?

Jbambamtex · September 19, 2022

King_strut, did you ever find the problem?

I have the same issue with a 2012 Tahoe 5.3. First start of the day is usually normal, almost every start for the rest of the day has a long start, no tach, and loss of power. The only exception is if it sits for 4 or 5 hours, then maybe it will start normal. I have been trouble shooting for a couple of months and find nothing.