hey.. I can't set my timing. This 1991 truck has an erratic idle, like it's missing on one cylinder occasionally. I followed procedure (grounding 2 sockets in diag. plug), and the timing light flashes erratically between 5 and 12 degrees. All new ignition parts and a TPS didn't solve it. Any ideas?

 

Some jumping in regard to timing is normal, it's a gear driven system on top of a chain.. It's not an exact science... Is it really all over the map when jumpered? Holding a constant RPM?

 

it's erratic.. between 4 and (roughly) 20 BTDC. i noticed that it holds a steady reading once it's slightly off idle. i wonder if the TPS base setting is screwed up..

 

Bingo, you are right on. The TPS's sole job is to change timing and also turn off injectors on decel. Your "IDL" contacts are openening too soon, this bumps timing. A good way to tell if the TPS is out of adjudstment is to install the jump wire. You want to hear the engine RPM's drop. If not, TPS is out of adjustment. And usually the TPS needs to be moved CCW just a smidge. First, make sure the throttle blade isnt gummed up and not allowing it to fully close. If this is all in order, a secret shortcut way of fixing it if your are close is to install the jumper, have it idling, loosen the top screw (the bottom cant be loosened without TB removal, so leave it as is) and now with the end of a screwdriver handle tap the TPS in a CCW fashion by tapping on the connector. Do this till the idle comes down (IDL contactls close) and while its still running, tighten the screw. If the idle jumps up while tightening the set screw, tap it a smidge further CCW. When its all done, jumper still in and idling. Watcht her timing mark with the light and a small amount of throttle will open the IDL points and you should see the timing bump right up, and at idle, coem back down and be rock steady!

 

Yep, test and adjust the TPS:
- http://www.4crawler.com/4x4/CheapTri...roubleshooting

 

The TPS tested BAD. I got one last night but haven't had a chance to put it in yet... Thanks very much for the great descrition of how to adjust it!! I'll update when it's in. -Karl