P1518 Code, Engine WON'T SHUT OFF!

[monomoit]

Greetings all, my first post here, but it should be an interesting one... I have a 2003 Yukon XL, 106,000 miles, that just had a replacement engine installed (of unknown mileage). I've been running it trouble-free for about a week (400+ miles), when all of a sudden, today, I had a problem.

Drove the truck for about 2 miles (started cold), and parked it for about 30 minutes. When I went to restart it, I was greeted with a Check Engine Light and the messages "STABILITY SYS DISABLED" and "REDUCED ENGINE POWER". Drove the 2 miles home at less than 20 miles per hour. Yes, engine power was indeed reduced...

Plugged the scanner in and got a P1518 code: "Electronic Throttle Module to PCM Communication". I did not clear the code, but when I went to restart the truck again, no CEL, no messages, and no reduced power - drove it 1/4 mile down the road, and it ran fine.

An hour later, when I started the truck, the CEL was back, along with the previous messages, plus "LOW COOLANT LEVEL" and "SERVICE STABILITY". The throttle now had zero response. This is the point, however, at which things got REALLY interesting; when I went to shut the engine off, it didn't. I turned the key, pulled it out of the ignition, and the truck just kept running, with the dash lights flashing. It took multiple attempts at turning the key on and off before the engine finally shut down.

For reference: the coolant in the surge tank is at the correct level, engine temperature was COLD during this event, battery voltage is fine, and no codes other than P1518 are in evidence.

So... any thoughts on what on earth could be causing this?!? Any help would be greatly appreciated.

 

[monomoit]

Well, sorry that there were no responses to this, BUT - problem solved! Ground connection G103, behind the passenger side cylinder head, was not even finger tight! The mechanic who installed the motor simply forgot to torque it. Given how loose it was, I'm astounded that the truck EVER ran. But, the connection is tight now, and everything seems to be working fine...

 

[Grimes]

Just for future reference, how did you discover that that was the problem?

As well, to anyone, what ways are there to safely turn off the engine in the event that turning and removing the key doesn't work? I'd deuce a brick if that happened to me. I'm very glad for you that simply doing it a few times eventually had it turn off correctly

 

[monomoit]

Grimes, from the research I was able to do, this problem appears to have two primary causes: the throttle body connector/wiring, and the throttle body ground (G103). I have to admit, though, few of the instances reported had the same set of symptoms as mine! Both of these potential culprits could be easily attributable to the engine recently being swapped.

Because the throttle body connector is "right there", it was pretty straightforward ruling this out as the cause. Locating the G103 ground, however, was a little bit more of a challenge, involving inspection mirrors and lights. Once found, accessing it wasn't too bad, made easier by removing the engine cover, and using a series of drive extensions with a universal and a 13mm socket.

There's absolutely NO question that this was my problem. As stated in my previous post, the bolt wasn't even finger tight. I'm still astounded that the truck ran at all.

As for safely shutting off the engine in the event that the key doesn't work - the negative battery terminal would take care of that straight away. I was about to grab a wrench myself, when the key finally worked.