Engine Speed Flair @ Start: Brief High RPM

[RAVC1]

I am observing a high/very high engine speed when I start my 88 Xj lately. This has been an issue and is becoming more common. At engine start my engine tachometer has been as high as 3k rpm; sometimes for many seconds.

Note I converted my 88 Xj from AW4 to an AX15 last summer (22 mpg is a beautiful thing!). I have added an engine/firewall ground that is an old Xj negative battery cable (#4 welding cable?) in order to address the engine ground issues. I also cut off (3"-4") and soldered new ground connections at the engine oil dipstick mounting stud. Presumably, my grounds are in a good state. However, I have not yet removed C101 and directly soldered all connections through that connector as advised here on during discussions regarding no starts & CPS issues this January.

Last week I decided to open the engine hood during one of these "extended high idle starts" and disconnect the TPS connectors. The square 4-pin connector did not change my engine rpm (I think the 4-pin connector communicates with the AW4 controller which is removed and not connected; at the time of AX15 conversion I jumped two wires in the trans harness - this method is documented well here on if you retain the AW4 harness - to provide a start signal to the relay; otherwise no functions associated with engine start occur.). However, when I disconnected the 3-pin (to Renix) the engine rpm decreased rapidly and fell to a normal 4.0L engine idle speed. I did not touch the IAC motor connector at this time.

Yesterday, I experienced yet another long high idle and disconnected both TPS connectors and the IAC connector as well. The engine idle speed remained elevated near 3000 rpm until it decided to fall to a normal level nearly a minute after the engine started. This tells me my TPS is not the issue.

Can anyone offer insight/interpret these results better than I?

Thanks,

Rick

 

[yenkoz]

It could be that your IAC motor is hanging up. Try unplugging your IAC motor when it's idleing at low rpm. Then after it sits for a bit fire the engine up and see what it does. Also you could check with a volt meter and see if the IAC is getting a good signal from the computer. Being able to plug it into a scanner and see what the computer is trying to do would help a lot.

 

[Spearfisher XJ]

IAC?

 

[old_man]

IAC is the idle air controller mounted on the throttle body.

Personally I think your B+ Latch Relay is probably bad. It holds the power on the computer for a couple of seconds after the switch is turned off, allowing the computer the time to reposition the IAC and get it ready for the next start. If it doesn't, when you start the engine revs for a few seconds until the computer can readjust the IAC.

 

[RAVC1]

My initial suspicion was poor IAC motor operation as this is how this vintage of Jeep engine starts; that is, throttle is not involved. However, I am unable to determine if the reason for this is anything other than the IAC motor.

What you are telling me is the B+ latch relay maintains power on the ECU for a brief period after the ignition switch is turned off allowing it to position the IAC. I will look at the electrical schematics in my FSM to interpret this further. You make a good point; I just do not understand it yet or how to test the B+ latch relay.

Thanks,

Rick

 

[cruiser54]

You may also have a faulty TPS. Check the stuff already mentioned first, but TPS can cause high idle flare.

 

[RAVC1]

Progress has been made but I'm uncertain if I have put this issue behind me. The work I have performed since my IP is:

1. New OEM rotor,
2. New OEM cap,
3. New Magnecor #6044 8 mm wires,
4. New Champion #7346 Double Platinum plugs RC10PY4,
5. Repaired each of the four connections to the ICM. Most were sound from a mechanical perspective but insulation had cracked leaving the copper conductor exposed to the environment. The ECU square wave wire at ICM connector #2 was horrible. A loose crimp connector that must have been installed at my request to have my old car alarm removed in entirety.
6. A B+ relay change from my 1989 Xj parts vehicle; used 12 VDC relay but new to my 1988 Xj Limited for this purpose.

All starts have been much better controlled; typically 2000 rpm max and brief speed flairs. With that said, my wife informed me she experienced a lone brief 3000 rpm flair Saturday afternoon after much of these improvements were made. The repairs to ICM connector #1 and the Magnecor wires were installed after this lone 3000 rpm start. Since the battery has been disconnected twice in that time the Renix may not have sufficiently "learned" its new values (i.e., I was told several years ago driving 15 minutes above 45 mph is required to reprogram the Renix properly after the battery is disconnected.

Thanks for your input.

Rick

 

[cruiser54]

Glad to hear of your success. If it continues, look at the TPS.

Have you refreshed all your grounds yet?

Renix doesn't learn anything BTW.

 

[RAVC1]

cruiser54,

Yes, the TPS will be my next step. My initial post discusses the grounds; I'll summarize again here:

1 - head/firewall ground improved to a spare Xj negative battery cable. I soldered copper lugs onto each end and sealed each with heat shrink tubing.

2 - smaller spade grounds (are there 3 in number???) at oil dipstick mounting bracket high on passenger side of block; cut 2-3" off each and soldered new wire/terminals on.

These are the only grounds I currently know of on a Renix Xj.

Renix doesn't learn values? Now I am confused; I assumed the 15 min/45 mph drive was for that purpose. A dealer technician gave me those instructions many years ago. What is the purpose of this 15 min/45 mph drive then?

Rick

 

[cruiser54]

OBD1 might have learned but Renix doesn't. I was service manager/shop foreman at the local dealer from 1980 through 1992. Well known that Renix isn't adaptive. Sorry I had a brain fart about your grounds. Responding to too many posts I guess. Your TPS ground wire may not be grounding well through the harness. Some people add a ground wire from the TPS directly to the intake manifold area. TPS also had a high failure rate. It can be checked and adjusted first before replacing it.

 

[RAVC1]

Okay. This simply means I misunderstood what I was told nearly 20 years ago; this is not the first time.

I do appreciate you mentioning the direct TPS ground as I had forgotten about that. After I converted this from the AW4 to the AX15 I have had no TPS-related start issues; only the CPS issues I faced earlier this year in January. Oddly, those stall/subsequent no-start issues were resolved by placing a new Metri-Pak male terminal on my original CPS and re-installing it. Apparently, something with the new Echlin CPS I purchased to perform my conversion was not right. I have not made an additional effort to re-test the new Echlin CPS.

Rick

 

[cruiser54]

I'm thinking of adding the TPS ground on my 88 XJ and 90 MJ just so I know it's done.

A thought. Have you ever cleaned out the C101 connector on the firewall above the brake booster? Lots of important electrical signals run through there. 15 minute job.

 

[RAVC1]

The advice I received this January regarding C101 were to remove it completely and patch each of the connections involved directly by soldering or a known good crimp tool. I have never heard of cleaning the connections in C101 for benefit.

Rick

 

[cruiser54]

Have you hardwired it yet? If not, you can do the cleaning. It was a suggested repair from the guys at JeepTech. I don't remember if it came out in bulletin form.
Unbolt the 2 halves. Clean out all the black tar crap with a pocket screwdriver real well. Flush both halves with a good electronis cleaner and dab it out about 3 times. Then, with a small pick, tweak the female connectors so they'll grab the pins more tightly. Add dielectric grase and bolt it back together.