350 Timing Troubles/Questions
 

With any luck someone here will be able to help lick this problem...

I'm having timing trouble again with my '88 V30 square. Engine is a 350 with a TBI and ECM from a '91 3/4-ton Cheyenne.

When I changed the EMC and TBI a year or so ago, I set the timing to 4deg. Should be running stronger, but better than how it ran at 0deg.

Serviced the truck a few days ago (including cap, rotor, O2 sensor, plugs...) and it ran worse. Checked the timing and it was up at 12deg, brought it back to about 4deg. Ran better, but when going up a hill it gets to a certain point where no matter how much you push the gas, there's no increase in power. Brought it down to 0deg, same deal.

Went on a small road trip to pick up tractors in NH today, ran with the same symptoms, power just wasn't there.
Got back home, brought the timing back to 4deg. Have yet to drive it again.

When I set it back to 4deg and plug the wire back in, the timing mark moves about 1" past the degree indicator. And watching it with the light, it stays in that exact spot from idle to high throttle. If you goose the throttle it will waver quick and them return to that spot 1" past the indicator.
Is the mark supposed to move as the throttle increases (as I assume a vac advance distributor would do)? Or does it move per the ECM only when the engine is under load?

I've posted here before about this issue and have been chasing it ever since the truck went on the road in 2011 after restoration. Its never been consistent on running properly, always runs boggy. Sometimes you'll have a good day and it'll run strong, but it always goes back to this. Just wish I knew what the heck was wrong :banghead:


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Re: 350 Timing Troubles/Questions
 

My non-expert guess is the computer is in "limp home mode". Basically a very tame setting that will start and run but little else.

I can't see that a TBI and ECM change the laws of engine physics that say a 350 wants a lot more advance then 0 or 4*.

Did all of the O2 and other sensors get moved? One or more missing signals might cause the computer to assume something is busted.

Re: 350 Timing Troubles/Questions
 

Never gave much thought to limp mode... good idea. Would make sense with how it's running.

Moved as in re-located on the engine or just checked? Everything is still in its place and all the sensors/modules have been changed within the past 3+/-years. I suppose I should start checking sensors and make sure they're all putting out something.

Aside from the large firewall plug and the plugs that go directly into the ECM, is there any more in between that are crucial? Maybe there's a bad connection along the way someplace?

Re: 350 Timing Troubles/Questions
 

Been a while since I have set timing on a tbi I think there's a tan wire you have to unplug it may have a black or white tracer its on right side (pass side) around engine harness just a thought worth checking

Re: 350 Timing Troubles/Questions
 

My only computerized vehicle is an 2009 Escape. The manual for that mentions Limp Home several times. Where I got the idea. I think there is also a 'open loop' mode if enough of the signals aren't getting to the computer. Like limp home, it is a basic tune for the TBI.

Don't overlook the ability of rodents to make after installation modifications to a wiring harness or rubber tubing.

Might check the ECM specs and see if it is expecting some signals from in cab wiring that might not have made the transfer trip.

Re: 350 Timing Troubles/Questions
 

Have you checked your MAP SENSOR.

Re: 350 Timing Troubles/Questions
 

Yep, that's the wire I unplug whenever I check the timing. Drivers side on this truck.

I'll have to do some reading up on the ECM and trace some wires when it warms up a bit end of the week. Driving around this AM it sounds like the spark isn't advancing, going to have to see what modules/sensors control spark advance.

MAP sensor was replaced June of '17 about 8,500 miles ago. Wouldn't hurt to check again...

Also... doesn't help that when my dad built this truck, they used about 8-10 squares to do it from junk yards. And of course, never kept the original harness and electronics to put back in. TBI from one truck, ECM from another, harness from another etc.

Re: 350 Timing Troubles/Questions
 

Is there a scan tool you can use to watch the sensors? Might help to know what the ECU is seeing.

Re: 350 Timing Troubles/Questions
 

We do have a couple of code readers. Might have a scan tool too now that I think of it. That would be great to go for a drive and see what's happening.

Just for kicks I had someone put the truck in gear and hold the brakes, with the timing light hooked up just to watch the mark (wire still plugged in), it wants to bring the mark towards 0 when accelerated under a load, VS. advance the timing.

Edit: Forgot to add... Checked the TPS and MAP sensors with a meter, voltage looks good. Also checked/tried swapping the square ESC block on top of the manifold, seems okay/no difference.

Re: 350 Timing Troubles/Questions
 

The timing should be less advanced with a load. There are 2 main things that effect what the timing should be - RPM and fuel in the cylinder.

As RPM goes higher, you need to ignite the fuel sooner, so it is pushing down at the correct time. In a mechanical distributor, this is handled by the mechanical advance.

As the load goes up, there is more air and fuel in the cylinders. This ignites faster, so less advance is needed. On a mechanical distributor, this is handled by the vacuum advance. Typically adding around 10* at high vacuum, and 0 at WOT.

Some numbers from my truck with points. These seem to be somewhat standard for a good tune on a mechanical distributor.
16* base, 18* mechanical, 10* vacuum, all in @ 2500 RPM.

At idle, it will see 26* (base + vacuum).
On the torque converter, it should see around 20-25. Depends on the stall speed and how hard you are on the gas. (base + a couple degrees of mechanical and vacuum)
Cruise around 2k RPM, around 36* (base + some mechanical + vacuum)
Cruise around 2500 RPM, 44* (base + mechanical + vacuum)
WOT @ 2500 RPM, 34* (base + mechanical)

I would imagine, in the end, your numbers should be similar. If you converted your engine to carb and threw and HEI on there, this is about what you would be tuning for. I would be interested to see what a stock truck has for timing. It might even report in the scan tool what it is expecting.