Renix 4.0s distributor timing

Does anyone know how robust the Renix system is on these? For instance, since the Renix system controls ignition timing, advance, retard, if one installed a distributor exactly 180 degrees off, would the Renix system figure this out, compensate and try to run the 4.0? How far off can the basic distributor timing be before the Renix can not compensate enough to start and run the engine?

Mike McGinness
South Houston, TX

The engine will not run with the distributor 180 out.

The distributor has to be "indexed". This is a mechanical timing problem, not an electrical problem. The rotor has to be pointing to the terminal in order for the spark to jump the gap. That means that the orientation is critical. Being off by even one tooth will effect performance.

In order to index the distributor correctly, rotate the cranshaft while the #1 plug is pulled. The correct "compression" stroke can be determined by air escaping. Set the timing to about 12 degrees before TDC using the timing scale on the front cover. Then insert the distributor and rotate the body so that the bolt hole aligns up. The rotor should be pointing to the #1 electrode. If it is not, pull the distributor, rotate the shaft and oil pump, then reinsert it and repeat the process until the rotor points to the #1 terminal.

Remember that the oil pump has to be rotated to align with the bottom of the distributor shaft, or the the distributor can not be fully inserted. A long flat blade screwdriver can be used to rotate the oil pump.

Thanks, I was going over the distributor replacement procedure (I have a 1987, 4.0) and I recalled doing every step properly except for possibly verifying that I was at TDC on the compression stroke and not the exhaust stroke on #1. So I starting wondering if I had screwed up. Since it runs reasonably well, that can not be the elusive answer to my low power in gear and low mileage then.

Thanks!

Mike McGinness
South Houston, TX

yeah buddy!

It's an interesting question, though. Dealing with the ignition system alone*, if you install a distributor exactly 180 degrees off (which would be at the TDC mark on the crank, but TDC for cylinder #6 instead of #1 in the case of the 4.0) it will work as long as you also then install the plug wires 180 degrees off. I've done this on other vehicles, and unless the distributor has some odd asymmetrical firing pattern, like an early 90 degree V6, or some VW's and old Porsches (some of which a slight timing variation between the two banks), the engine doesn't know the difference. It's the spaces in between that don't work.

Remember that the timing of the spark itself is only one function of the distributor. It must also send it to the plugs via the rotor, and that timing is fixed entirely by distributor position. Beyond a certain range, it just won't line up. In addition, the range of compensation the computer can provide is limited. I'm not sure which factors came into play, but when I experimented with timing on my 87, I found that it could not work well even if the distributor was a single tooth off. It would run well one tooth advanced, but started very hard. It would start well one tooth retarded, but stalled with throttle.

*As I said, I know that this works on old carbureted engines, but all bets are off for MPI fuel injection. While the ignition system might work perfectly 180 degrees off, it probably would work very poorly on any fuel injected system which, like the Jeeps, uses something other than the distributor to time the injector pulses. If the injectors are still firing 153624 while the distributor is firing 624153, the injection pulses which are supposed to occur just before the intake valve opens will occur instead just before the power stroke. Fuel injection will always be two strokes late, and I'd expect mixture problems.

ETA looking at the question that gives rise to this thread, I would strongly advise double checking the timing, since it's quite possible the engine would run "reasonably well" with the distributor wrong, but that mileage and power would suffer.

Mine was originally off one or two teeth as I recall and it ran, but poorly. Since I had other problems (so the Renix was runing in open loop mode, no coolant sensor and bad O2,...) at the time they may have compensated for the basic timing error as I think it was off 2 teeth, but its been too long to recall for sure.

As I recall I set the first cyclinder at TDC with the spark plug removed and turned the crankshaft manualy with a socket/wrench, following matching instructions from 2 or more sources, I worked it in and out until I got the rotor and #1 plug wire lined up correctly with the oil pump engadged.

I was rereading looking over things I did the last 2 years to see if I missed something, and 180 degree exhaust stroke got me to thinking and wondering if I did have the compression stroke or not. Seems like I must have done it (set the timing) right as I did not swap the plug wires 180 off and it runs pretty good, especially out of gear in park where it acts like a new engine/system.

So it's back to digging through sensors and wires on the Renix ECU and TCM for me to find the onboard gas and power thief in the higher gears (2-5).:read:

Thanks,

Mike McGinness
South Houston, TX