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Another high idle at start up thread.

O

outlander

NAXJA Forum User
Location
Columbus,Ohio
I've read alot of posts on this and alot of people say that the tps is causing this or that the iac is bad.I have the dreaded 2000rpm idle surge at start up on my 89 xj and have developed a theory that needs a little validation.

I'm thinking that the problem has something to do with the latch circuit on these older 4.0's.The latch circuit is responsible for positioning the iac properly for the next start up.So if the latch circuit is not functioning properly the iac is too far out of the tb resulting in the high idle at start up so many of us suffer from.

While working on the rig today I was looking into my high idle problem.Started it up and idle surged to 2000rpm then settled right back down to a normal steady idle.I unplugged the iac while it was in the right position with the jeep running.Turned of the engine started it again and no more high idle at start up!

My tps is not the problem...it tests out fine and the iac is functioning properly so I'm looking into the latch circuit as the problem.

Just thought I would start a thread on this while I'm working on the problem.More to come as I dig deeper into the latch circuit.
 
Your on the money as far as the latch relay and circuit. Its just that relays are so simple they don't usually fail. (looks like yours might have) So most go after cleaning the IAC, or replacing the IAC. All the relays are the same on a renix panel. Swap one and see if it fixes your problem. Good Luck.
 
jeepdeepfreak said:
I've read alot of posts on this and alot of people say that the tps is causing this or that the iac is bad.I have the dreaded 2000rpm idle surge at start up on my 89 xj and have developed a theory that needs a little validation.

I'm thinking that the problem has something to do with the latch circuit on these older 4.0's.The latch circuit is responsible for positioning the iac properly for the next start up.So if the latch circuit is not functioning properly the iac is too far out of the tb resulting in the high idle at start up so many of us suffer from.

While working on the rig today I was looking into my high idle problem.Started it up and idle surged to 2000rpm then settled right back down to a normal steady idle.I unplugged the iac while it was in the right position with the jeep running.Turned of the engine started it again and no more high idle at start up!

My tps is not the problem...it tests out fine and the iac is functioning properly so I'm looking into the latch circuit as the problem.

Just thought I would start a thread on this while I'm working on the problem.More to come as I dig deeper into the latch circuit.
Click to expand...

This is EXACTLY what my 88 does. I unplugged my iac and have been running like that for a few weeks now. What is a latch circuit. I have never heard of it before. Thanks.
 
The Latch relay is up on your passenger side inner fender. you have 4 relays there, IIRC, the latch relay is the one closest to your firewall. Its a simple relay just pops out and another one goes in, All 4 relays are exactly the same on that side, they all just operate different things, If you have a manual I think they even call it a "relay center" and will show you which one is the Latch relay. The circuit would be the wires from that to the IAC and ECM.Good Luck
 
88XJSPORT:

quote from my above post:
"The latch circuit is responsible for positioning the iac properly for the next start up.So if the latch circuit is not functioning properly the iac is too far out of the tb resulting in the high idle at start up so many of us suffer from".

Another thought on this is the relay could still be good but there is corrosion in the wires of the latch circuit which lowers input voltage to the iac causing it to be "slow".

I'll be checking all this shortly if the t-storm holds of today.I'll report back with any findings.If I can't find anything I'm thinking about leaving the damn iac disconnected....this high idle at start up on a cold engine has to be detrimental.
It starts and idles fine with the iac disconnected it makes me wonder what the hell the purpose for it is in the first place,it's supposed to control idle but with it disconnected my idle is fine.I suspect that a change in out side temp(like winter)would cause me to have to "reset" the iac's position in the throttle body by plugging it back in at idle so the ecm can tell it what position to be in...
 
When you crank your engine the ECU sends a number or Pulses to the IAC based on Air tem and Coolent temp.
If one of those is off it will open the IAC to far every time. When you turn the engine off the Power latch relay keeps the ECU on long enough for it to return the IAC to closed position. It needs to start from zero so the computer know where it is, there is no position feedback.
Your IAC and latch relay sound like they are working fine.
I would look at temp sensors or a vacuum leak.

TIP if you have someone turn the engine off while your under the hood, you will hear a buzzing sound for a couple of seconds coming from the IAC as it's pulsed back to (0)

Flinks.jpg
 
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