Renix Observation/Question

My '90 XJ is running like a fine Swiss watch but I have noticed one thing that puzzles me a little. I would love the thoughts from some of the experts out there. I should clarify that I implemented all of Cruiser54's recommendations and I doubt that there is a '90 XJ in existence with better grounds than this one.

The issue appears as a high idle immediately after start but this doesn't seem to be the same as previous threads because I can completely eliminate the issue if I simply leave the key in the "on" position for 4-5 seconds before I crank it. If I turn the key immediately to the crank position, it will fire right up but I will often see the idle jump to over 2000 rpm and will then want to settle down around 1200 rpm (in Park) and will idle at 800-850ish in Drive. If I show a little patience and wait the 4-5 seconds in the on position, it fires up and the idle drops almost immediately to 800 rpm and it will idle nicely around 450-500rpm in drive.

I am starting to think this is normal because it works like this every time. It is almost as if the reset of the IAC gets interrupted with the quick crank and it considers that wherever it stopped must be base setting. On the other hand, if i give it enough time to fully reset, it happily runs at a proper idle.

Thoughts? Is this normal or should I start looking for a fix?

I should add that when I restored this old girl a couple of years ago, I did a complete clean up of the throttle body so I know that there is no buildup in the IAC area.

How long since you cleaned the throttle body?

Should be done every 30K, just like plugs, wires, cap and rotor.

1037 miles since it was last cleaned. Challenge is that that was in 2014. It only gets driven about 20 miles per month (Pretty consistent) I guess i could have some varnish build but it should still be pretty good

cruiser54 said:

How long since you cleaned the throttle body?

Should be done every 30K, just like plugs, wires, cap and rotor.

Click to expand...

There is I am told a timed shut down relay (I forget the Renix name for it) that resets the IAC after the key is turned off by keeping the ECU powered up for about 5 seconds after shut down. I am not yet sure I believe that, as if the engine is hot at shut down, I would think it would need to be reset for a cold start before cranking. The ONLY thing I think could change in that 4-5 second delay from run to crank would be the IAC getting reset to lower the idle. For grins try disconnecting the IAC before you turn the engine off, and start it. Shut it down, then reconnect the IAC and restart immediately. Do it cold start, and then again once it has warmed up. Try some variations of that and post back the exact steps, sequence and results to see if we can get any more clues.

tjmotter said:

1037 miles since it was last cleaned. Challenge is that that was in 2014. It only gets driven about 20 miles per month (Pretty consistent) I guess i could have some varnish build but it should still be pretty good

Click to expand...

That's not an issue then.

ASD relay.

old_man said:

ASD relay.

Click to expand...

Different name was used on Renix, B+ or power latch relay is the Renix name.

Renix MPI manual says it it is a 3-5 second delay power on to the ECU to reset the IAC. That means the ECU makes some kind of start up guess for the setting. No details on the guess process.

Thanks to all!! I will start looking for this once I get back to the XJ next week.

Ecomike said:

Different name was used on Renix, B+ or power latch relay is the Renix name.

Renix MPI manual says it it is a 3-5 second delay power on to the ECU to reset the IAC. That means the ECU makes some kind of start up guess for the setting. No details on the guess process.

Click to expand...