Tech Q&A - April 2014

Kevin McClelland Feb 16, 2014 0 Comment(s)
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Silent Running

Q. I was wondering when GM quit using the fiber timing gear. I have a 1994 GMC 1500 with a 350 eng. I have put new plugs and new wires, new rotor, new cap, new fuel pump, new fuel filter, new EGR regulator, and the timing is right. It will start right up and pretty much idle perfect, but as soon as you put it in gear, it will hardly pull itself. So was wondering if this still had the nylon timing gear and thinking that it could be worn down. If you have any help at all on what I need to do, would be greatly helpful.

Mike Bergen
Via email

A. Back in the day, as a line mechanic at Muller Chevrolet, replacing timing sets was a regular occurrence. Why this is titled "Silent Running" is because these nylon/aluminum camshaft gears would leave you stranded. You couldn't hear them run because they weren't! If the owner was lucky, the engine would diesel on key off and the chain would jump a tooth or two. With the very mild camshafts in the production small blocks you could get away without bending the valves. Now, if the vehicle owner's son was driving the car, the valves were always bent. Not only did you have to tear the front of the engine off and replace the timing set, off came the heads for a valve job and several replaced valves. It was not only the small-block Chevys that had the benefit from the silent running timing chain sets; the Pontiac V-8's of this era were equipped with the same technology. For as many jumped timing sets we replaced on small-block Chevys, we probably replaced twice as many Pontiac sets. Great design!

From all indications the composite timing sets (Nylon and they also used a Bakelite material) ended in the mid 1980s and they went with straight steel camshaft sprockets. The Nylon was used for quiet operation and I think that the marketing name was something like "Silent link." Your 1994 vintage small-block most certainly has a steel timing set installed. I have a couple of questions. You have definitely replaced a few parts chasing down your problem. Any or all of them could create the condition of poor performance. You said that you had checked the timing and it was correct. Your truck is equipped with electronic spark control. To check the timing correctly you must disconnect the distributor from the computer to set the initial or base timing. I believe that your base timing is zero degrees in '94. There is a tan/black wire that has a weather pack connector in it to disconnect the ESC and allow the engine to run on the distributor ignition module. This will allow you to set your base timing. The timing connector has been moved from the engine compartment to inside the cab. This new location is not currently reflected in the Service or Owner's Manual. The new location is under the dash directly below the glove compartment. The tan/black wire is routed for a short distance outside of a large wiring harness. This large harness is routed between the heater blower motor and evaporator case. The timing connector is taped to the wire bundle. Once the base timing is set correctly, re-connect the tan/black wire and the timing should jump up into the 20-degree range at idle.

From your description of the lack of performance and that when you put the truck into gear it falls on its face, sounds like a timing issue. If you were setting the initial timing with the computer connected, it would only retard the timing more when you drop it into gear. Hopefully, this gives you a direction to find your problem. Good luck.

Rock On

Q. Your column is the best. It's the first thing I look at when my magazine comes in the mail. I'm always learning something new and interesting. I was reading a past (August 2009) column about GM Performance Parts 1.6 ratio rockers (PN 12370839 for kit) and was wondering if these would fit my truck? I have a 2003 S10 with a 4.3L Vortec V6. Would they fit under my stock rocker covers? If they fit, do you have a part number for a single rocker since I don't need 16 of them? Thanks for your time.

Todd Yoshioka
Honolulu, HI

A. Todd, thanks for the props. We try to help as many folks as we can with our answers. If we spread the info around and make it different every month, it will stay interesting. The 1.6 ratio rockers that you listed from the August 2009 column have been discontinued as the original supplier went under. These rockers were exact replicas of the 1996 LT4 aluminum roller rockers with 3/8 inch trunnions in place of the metric version that were in the production rockers. Not only did these rockers have a specific 3/8-inch trunnions, they were supplied with extremely short Posi-lock adjusters which would clear the factory valve covers on small-blocks, and your 4.3L V-6.

As you listed, the set of 16 were sold under PN 12370839, and have been superseded by PN 19210729. Our pal Ken Casey at Elway Chevrolet in Englewood CO. has the new rockers in stock, but they have changed the body from the original LT4 design. We would be concerned that they may not be a direct swap.

The single units were sold under PN 12367346. The interesting thing is that the single unit service part number is still active and GM has stock. You can order these directly from Ken at Elway. We've also found a limited stock of the original 16 set part number LT4 design in stock at Summit Racing. You may want to check the pricing as the set of 16 may actually cost you less than purchasing 12 of the service rockers. You may want to scoop these up as soon as you can because when they are out of stock, they will be gone.



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