All of a sudden...fast cold start! - but why?

OK, what gives?

Ever since buying my '93 XJ (4.0L H.O., 2WD, auto) 3 months ago it has
always taken a concerned amount of starter spinning and praying (or cursing)
to get the beast to fire up when cold.

I've changed the CPS (new), cleaned the NSS, new O2 sensor, and cleaned
every dang connector I could lay my mitts on - including the ECM connector.
I cleaned the pintle on the Idle Air Motor - but only superficially - as I had
read to NOT pull or twist the pintle while doing this.

Although this improved things a little, it nontheless still required two (2)
quick spins of the starter (and a silent prayer) to get things going.

That is, until today.

This morning, the temp happened to be in the very warm 80's, and dry.
I hopped into the car and flicked the key - only once - the starter whirred
2-3 revolutions...and the engine immediately burst to life!

I was pleasantly astounded.

To be sure that this wasn't a fluke, I lumbered the car back into my garage
where I fiddled with the rear hatch speakers for 3 hours. The previous owner
was an absolute "pinhead" as he had severed the stock speaker wire harness,
then connected the L & R rear speakers together (using household twist-on
wire nuts) - then he hooked the speakers up to the cargo dome light. ...DOH!

After sorting this out...I hopped back into the driver's seat and, again, after
giving it one shot of the starter, the cold engine immediately roared to life as
though it were a real, "normal", car.

I'm not one to complain about hitting the Lotto...but does anybody out there
have a theory on what could have caused this fortunate change of events?

Anybody know which onboard "gizmo" is especially sensitive to ambient
air temperatures, humidity, etc.?

MAP sensor? Maybe?

Sluggish pintle on the Idle Air Motor (due to gummed fuel, internally)?

Three things, that seemed to have the most affect on starts for me, are the distributor cap, the pressure in the fuel rail and how fast the starter motor is turning over.
Have noticed cleaning the carbon dust out of the starter, has helped with starts, also a fully charged/strong battery. When the pressure in the fuel rail has fallen some, from sitting (usually takes awhile) usually takes more revolutions to get it going.

I posted a similar problem a few months ago because I was experiencing the same situation with my 98 (Glad to see I'm not the only one). Only thing suggested to me was to wait for fuel line to fully pressurize before starting. It seems pretty random to me. When it's cold I'll wait a few seconds with the car "on" before starting and sometimes it will start right up and sometimes it will just crank. When it is just cranking, after a couple of seconds I turn off then turn on quickly , it fires right up. I'm thinking it might be something with the injectors or the computer control of how much fuel to supply during cold start. Since it has never failed to start, haven't tried to mess with it.
-joe

warm ambiant tempeture gets my vote. After owning two of these jeeps for several years, I got used to the fact that when its cold outside, she aint gonna start fast-an when its warm, she starts right up,right now.

your lucky with only 2 to3 revolutions. My magic numbers are 13 cold, 9 hot.

figure this- when its warm, the battery puts out more juice, so everythings getting what it deserves. The gas is more volitol being warm, the map sensor is happy an putting on sunscreen, and the 02 sensor is warm. Besides, half the gremlins have gone north for the winter- the other half are lazy in summer...enjoy!

Here's a follow-up on my fast-start mystery.

First, I forgot to mention that I had already checked the fuel pressure:
static, running, and leak-down. All = A-OK. (According to the manual,
it's spot-on perfect). The fuel pressure goes to specs immediately when
the ignition key is switched ON. There's no delay or waiting for pressure.

The distributor cap & rotor are brand new (brass contacts).

I keep my feet off the gas pedal when starting.

Later that evening, it was a relatively cooler 55º, and the car reverted
to it's usual fussiness - and I had to give the starter two spins before
the engine was able to fire up and go.

Then this morning, the temps was in the 80s. The engine was stone-cold.
I jumped in and flicked the key: ...wrrr-rrr-rrr...v-v-rrro-o-o-m-mm!!!.

Me suspects that maybe the warmer ambient temps is affecting something
on the car (ie: some sensor??) to tell the ECM to immediately "get things
going" when starting.

...or...

Maybe the injectors aren't misting the fuel into tiny enough droplets
so that the fuel takes longer to vaporize when cold?

I bought the car only 2.5 months ago and so I do not know
the history of the injectors that are presently on the engine.

Sound reasonable?

Anybody have a favorite brand injector to recommend?

Perhaps it is a condensation problem. I would squirt a little wd40 under your distributor cap when the ambient temp is cool and the engine is cold just to see if that makes an effect.

Here is something I am waiting an ansewer on Dragon from a earlier post .. It is driving me crazy ....

Quote:
Originally Posted by Wayne Sihler
The sensor for the temp guage(located at the back of the head)and the Engine Coolant sensor(located in the T-stat housing) are two different things.
The sensor in the T-stat housing is connected to the PCM,on later models(98 on)this sensor supplys both the guage and the PCM with info.
Are you sure that what you saw was the temp reading for the coolant or was it the temp for the intake air temp ?
Wayne

Hello All,
Reading this thread I may have stumbled onto something !! I have been having start problems in cold weather with my 92 XJ 4.0I6 .. I have replaced every sensor along with rebuilding my NSS. Still when the weather drops below 32 degrees I experience a no start condition , after turning the key on and off several times she attempts to start but floods out and by holding the peddel to the floor to blow out injectors and a cloud of blue smoke she will run all day with no problems .. Q: ? Could this be linked to my T-stat in my housing is faulty sending no signal to the pcm ?? I don't believe it has ever been changed 166,000 miles ...
Thanks in advance for you input

geepers

Just an oppinion, I did some serious fuel pressure tests, looking at another problem. Had a fuel pressure gauge tied to my windshield wiper for days. After shutdown, the pressure drops in the fuel rail, after sitting for awhile it drops some more. At start up on a cold motor, it typically takes me a count of a fast 5 or 6 to get her to pop. Even after leaving the key in the run position, long enough to repressurize the fuel rail (though a repressurized fuel rail does help some). Just a guess, but I´m thinking some of the fuel in the rail has turned to vapor, have to turn her over for awhile, to purge the vapor. Also think the RPM of the starter motor has some imput, either more injector cycles and/or pulse frequency from the CPS.
Next time I think about it, I´m gonna hold a rag over the fuel rail, test fitting and squirt a little fuel to purge the rail and see what happens.

Hey Geepers - I changed my temp sensor (the one at the thermostat housing)
because the electric cooling fan wouldn't come on.

I can say that the fan now works as it should.

It did not affect the cold start behavior, one way or the other, however.

On a '93, the rear temp sensor is the one that works the dash gauge
(or idiot light).