Well I tried to "improve" things by installing a new distributor cap and rotor, plugs etc. on my '98 with 122,000 miles. Now it barely runs. I had to completely remove the distributor as one of the cap mounting screws rung off so I had to pull the whole thing to drill it out and re-tap it. I was VERY careful about noting the location of the rotor shaft so I could put it (the distributor) back in the same position it came out so as not to affect the timing. It will crank up and idle fine but at about 8 mph it starts backfiring and acts like it tries to shut down if I attempt to accelerate beyond about 8 mph. The "check engine" light is now on and so a friend is going to bring over his code reader. In the meantime, any suggestions on what might be going on?

 

More than likely the fuel sync is off.

The distributor, camshaft position sensor, controls the fuel injection sync. The crank position sensor and the PCM control the ignition timing.

A good scan tool, not a code reader, will be needed to reset it. You need some live data.

 

Thank you for you reply. I hooked up a Cen Tech (I think it is a scan tool) and it spit out P1391 which seems to indicate an issue with the Camshaft position sensor or the crankshaft position sensor. I di find today where the #3 plug wire has a "break" in it. Could that cause the same code?

 

P1391 is an intermittent loss of the cam or crank position sensor.

It's probably not as a result of the break in the wire. That should trip a misfire code. Obviously you need to fix it though.

Make sure the connectors on both the cam and crank sensors are clean and tight. Are you sure you didn't hit the sensor with anything while you had the distributor out? You can also check the 5 volt supply. They both share the same supply/ground.

Not sure on a '98 but in '00 the 5 volt supply was the orange wire, pin 3. The black with light blue tracer is the ground, pin 2.

Pin 1 is the signal out on both.

 

I'd still wager that your sync is +/- 12 degrees off. Noting where the shaft is within that tight of a range isnt easy.

 

I agree with Magnet. I'd have the fuel sync checked before anything else - except the wire #3.

 

I'm not sure what you mean by the fuel sync? Please help

 

The PCM uses the crank sensor (mounted in the bellhousing) and the cam sensor (located in the base of the distributor) to determine how much the cam is advanced or retarded in relation to the crankshaft (due to timing chain stretch), and also uses this information on when to fire the fuel injectors and spark plugs. The allowed operational range is +/- 8 degrees of cam retard/advance, although up to +/- 10 degrees have been known to be drivable.

If you drop the distributor in at any tiny bit off of the original angle, it will make the computer think the cam is out of range, and cause the injectors to fire very early (backfire out the intake) or the spark plugs to fire very late (backfire out the exhaust). Among other things. But that's the general jist.

 

So Dan and Magnet is what your referring to as fuel sync, what most people just call timing? So is the timing off? Can I loosen the distributor hold down screw and manually turn it (like the old days) to 0 degrees TDC or whatever and fix the problem?

 

Yes and no.

On a conventional HEI motor, turning the distributor controlled your rotor phasing as well as the timing of the mag pickup to fire the coil.

On an EFI motor, you have a crank sensor mounted in the bell housing which reads the flywheel. The flywheel has 3 or 4 notches (6 or 8 cyl) that tells the PCM when each cylinder is at TDC. It controls nothing.

Also on an EFI motor, you have a cam sensor mounted in the distributor, which reads blades on the distributor shaft- it either has 6 or 8 blades (I think- never have had mine opened). It controls nothing.

The PCM reads the pulses from both sensors, and uses the "timing" of these two sensors to fire the coil and fuel injectors.

So yes, fuel sync is "timing". But not traditional timing.

In traditional HEI timing, you turn the distributor, and it'll start misfiring because the rotor phasing is off- the spark always goes forward because that airgap is decreasing on levels of calculus, but it also threw off the pickup, advancing the timing. So the blade on the distributor shaft stayed in phase with the rotor, but both lost phase with the spark plug terminal. So you'd crank in 20 degrees of timing, but the rotor was also now 20 degrees before the terminal.

MSD Ignitio fixed (or complicated) this by bringing in a crank sensor. Now, you could phase the rotor to the terminal by turning the distributor, and the crank sensor would fire the coil wherever you set it. Everything worked great.

Then EFI came along. (Posting for timeout reset)