1994 GMC Sierra 2500 hard starting/timing


steve in alaska
11-05-2010, 09:14 PM
I'm working on a 1994 truck for a friend. When cranking it over, it acts like it's too far advanced..he changed all plugs/wires/cap/rotor etc. I checked it over, and everything is hooked up correctly, proper firing order etc. I disconnected the timing connector in the wiring harness next to the distributor, and adjusted the timing to 0 degrees, per the underhood sticker. Funny thing is, even with the timing connector hooked back up, I see no change in the timing mark on the harmonic balancer when I rev the engine. It just stays at TDC. So, here's what else I've done. I checked the TPS, and it checks out fine. The MAP seems to be OK when I run the engine and have my scanner hooked up, voltage is about 1.5V at idle, and goes up to 4.6 when floored..BUT..with the key on, engine off I'm getting a reading of 4.5 volts, and my Chilton's says I'm supposed to get only 0.5 volts..could this be the problem? I checked it at the MAP sensor with a volt meter and came up with the same thing, back probing the green wire and the ground wire together. I also changed the thermostat (old one stuck open, engine only hit 128F tops by my scanner) changed the coolant temp sender as the gauge wasn't registering (it's OK now) and the coolant temperature sensor, as the OHM readings were way off on the original one. New one checks out OK.

Once I get the engine started (which takes some time since it's so far advanced!) , it idles perfectly..but will cough and sputter sporadically but sometimes smooth out..it also doesn't seem to have the "soup" it should for a 350CI, but I attribute that to the spark not advancing..

I have no trouble codes..I was thinking timing chain jump possibly, but if that were the case, I believe it would run rough at all times! Could it be the ESC (electronic spark control)? Or is the ESC part of the ECM?

I'm a pretty decent mechanic, but this one has me shaking my head. Anyone have any suggestions? Engine has TBI and the distributor has no vacuum of course..and if anyone could confirm the color and location of the timing connector I disconnected to time it, I'd appreciate it. My Chilton's doesn't show a picture of the location..and no, it's not one of those that hooks up under the plastic cover on the firewall. (disregard all the talk about the wire, see below)

Thanx all.

edit: after much research on the web, I guess the 94s had the timing connector wire, tan w/black stripe moved to the cab by the glovebox/heater motor...so I unplugged the wrong wire..I'll re-time tomorrow. But even with that screwup, shouldn't the timing mark change when the engine is accelerated?

EDIT/Problem NOT the timing chain!

So I finally found the timing connector..it was indeed in the wiring harness by the glovebox..someone had taped the connector up! So I unplugged it, and the truck started right up. However, to put the timing mark on 0 degrees, I had to retard the timing almost a full cylinder! It ran, but only at idle. It would start to miss as I accelerated. So I hooked the timing connector back up, and it would barely run! Just surged, and quit, etc. So I pulled the valve cover on the 1-3-5-7 side..turned the engine over by hand, until the number 1 exhaust valve was closing, and the intake was opening..which would be number 6's firing point (TDC) and I was barely past #3 cylinder's firing point. NOTE: this where I left off yesterday, and wrote thinking it was the timing chain. However, after a good night's sleep, I realized I had been looking at the ROTOR's position, and not the harmonic balancer. So I got up today, popped the valve cover again, and did the same thing with the intake/exhaust positions on #1 cylinder. So, at exactly the point the exhaust was closed, and the intake was opening..the harmonic balancer timing mark was TDC (#6 cylinder firing position.) So I'm back to square one. I checked the distributor pick up coil and the external coil by the book using an ohm meter and they check out. I also pulled the distributor and the teeth are like new, there's no slop in the bearing, or endplay. Like I said, it starts right up and idles flawlessly, up to about 1000/1500 RPM with the timing set correctly (TDC) and the timing connector disconnected. But if I hook that back up, the engine will barely run, just lopes..like someone is turning the key on and off again. I have checked all vacuum hoses and they're fine. See my comment about the MAP sensor above..could that be it? I unhooked the MAP sensor and there was no change..wonder if I should unhook the MAP sensor and plug the timing connector back in? What about the control module in the distributor? In my experience, they either work, or quit completely. And lastly, could it be the ECM? I'm not getting ANY codes (except for the timing connector disconnected). I don't want to blindly throw money at this. I really thought it had to be the timing chain, but like I said, the valve timing checks out.

Anyone have any advice?? Greatly appreciated!

2000CAYukon
11-12-2010, 05:39 PM
Make sure the MAP will hold a vacuum. Also make sure sure the vacuum port on the TBI is not blocked.

Did you try the other (known to be good) dist yet? Does the dist drive gear look worn?

//2000CAYukon

steve in alaska
11-12-2010, 11:47 PM
It was indeed the distributor! Swapped known good one in, and set it to TDC with timing light, then hooked up timing connector wire and it ran perfectly. No miss, runs smooth thru all RPMs, no hesitation or miss on acceleration. Now I have to figure out if it's the ignition module (which I'll swap out tomorrow) or the pickup coil. They definitely get corroded in this wet country. The bearings and the teeth do not seem to be worn at all on the original (bad) one. I'll post my findings when I finish up!

steve in alaska
11-13-2010, 11:18 PM
Well, hooray! Problem solved. It was the trigger wheel in the distributor! The serrations in the shaft were rusted and worn (as were the grooves in the trigger wheel) so the wheel would spin on the shaft, constantly changing timing. That's why it would run somewhat OK at times, and be way off at other times..mimicked a jumped timing chain. And to add frosting on the cake..I got a used distributor from a guy down the road..just swapped out cap rotor and ignition module..set it at TDC and reconnected the tan/black wire, and voila! Runs great! Hope this helps others, as it was driving me crazy!!

2000CAYukon
11-15-2010, 05:57 PM
Well, hooray! Problem solved. It was the trigger wheel in the distributor! The serrations in the shaft were rusted and worn (as were the grooves in the trigger wheel) so the wheel would spin on the shaft, constantly changing timing. That's why it would run somewhat OK at times, and be way off at other times..mimicked a jumped timing chain. And to add frosting on the cake..I got a used distributor from a guy down the road..just swapped out cap rotor and ignition module..set it at TDC and reconnected the tan/black wire, and voila! Runs great! Hope this helps others, as it was driving me crazy!!

Glad to hear you got it resolved!

//2000CAYukon

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