slow startin

88 XJ 4.0L auto trans

I know the older cherokee's start slow and all but im kinda curious as to why and im the process of figurin out the whole system involved in starting.

i put a timing light on plug 1, which is supposed to fire first. i cranked the engine and sure enough within a second the #1 plug wire had voltage. This means that the CPS and Cam sensor in the distributor are working relativly quickly right?

So if there is spark and it takes 3+ seconds more to actually fire, that leads me to believe there is a fueling issue. I am pretty sure the pump is good, and it doenst matter if you push in on the gas pedal to let more air in thru the throttle body. the reason i think the pump is good is b/c if i turn the key to the on position and let it sit for however long, 3 sec to 1 min, doesnt matter, it takes the same amount of time to start as it does going from the off position to the crank position w/ the key.

only things i could come up with is low pressure from the pump or maybe some kind of Vac leak at the pressure regulator. when i turn the key to the on pos. and the pump does its whine, it does cut off, and shouldnt that mean that the fuel rail is charged to the required value to shut the pump off via the ceramic resistor thing on the side of the driver side fender (ballast resistor?)

I have yet to get my hands on a schrader type fuel pressure checker to check fuel rail pressure. hopefully i will get that next week.

any ideas on what i have sofar. plugs, wires, distrib cap rotor and all that tune-up BS is new. jeep takes slightly longer to start when real cold out as opposed to hot.

They do start slow by design and the ECU is in control of that. I believe that it will not open the injectors until it is happy with the cranking rpm. It's not clear if you think yours is taking longer than normal to start. 3 seconds sounds about right.

My 87 takes about 3 seconds to start. If it always fires up, it ain't broke.

I was however able to make the starter spin faster by replaceing the battery cables with big ones made for tractors. This improved it by about half a second, but it still takes long enough that some people comment about it. It's a Jeep thing.

88 Jeeper said:

88 XJ 4.0L auto trans

... when i turn the key to the on pos. and the pump does its whine, it does cut off, and shouldnt that mean that the fuel rail is charged to the required value to shut the pump off via the ceramic resistor thing on the side of the driver side fender (ballast resistor?)

Click to expand...

NOPE!, the fuel pump shut-off is a time-based thing -- the ballast resistor slows and quiets the pump in normal operation.


Slow cranking and low compression are good places to look for really long crank-times after you've eliminated some of the normal things (try measuring the actually key-to-start times with heavy and thin oils, I've seen increased start-times with just a new oil-pump robbing the cranking speed).

3-seconds isn't too bad - if you consider that the "spark" that you're seeing on #1 has to have an injection pulse ~436* earlier to have loaded the cylinder -- actually, when you see that first #1 spark the #2 piston is 120*BTDCe and it's injector will fire at ~76*BTDCe -- as the first cylinder fueled it's still 475* or so from gettin first spark into a fueled cylinder (assuming that theintake manifold is flowing sufficiently to draw anywhere near a stocc-mixture into the cylinder - probably'll be lean for at least qty=2 or 4 more cycles)... and that that happens once you've got a fairly stable cranking speed (best results are above 9revs per second ~550 rpm cranking speed)...

Have you investigated for proper operation of the Idle Stepper Motor (retracted after shutdown and fairly well extended under running idle conditions)?

Also, how about looking at the Intake Air Temp and the Coolant Temp data -- are those accurate? (a lower than expected resistance on either of those will cause you to try to start leaner than necessary).

Injector spray patterns and spark performance (gap, index, etc) also contribute