Explain these sensors to me

I have spent hours reading these forums about the starting issues.

The problem: when i get in the jeep after it has been sitting for awhile, it has a really long, lagging start on the first key turn. when i try to crank it the 2nd time, it fires right up.

I've read and searched, and read some more and there seems to be alot of different ideas about what causes this. What do these sensors do?

Camshaft Position Sensor (replaced a few months ago)

Crankshaft Position Sensor (replaced today)

MAP Sensor

TPS

Today i replaced the crankshaft position sensor, it didnt seem to affect the problem any.

I replaced the camshaft sensor a few months ago, it didnt fix anything either. Also the oil pressure/pump sending unit was replaced because it was cracked.

I also went out and sat in the jeep for about 30 sec with it keyed in the on position, giving the fuel pump alot of time to prime, and it still has a very lengthy start on the first key rotation.

Ideas fella's?

Do i just need a new starter...as far as i know it hasnt been replaced. Jeep has 86K miles.

I found a post. Jon is an "expert" in my books, give him a pm. He will be more than happy to help u out. This thread will give you an idea and an explanation as to why older Renix motors take so long to start. Its normal, mine takes up to 5 seconds every once in a while, no biggy

http://www.naxja.org/forum/showthread.php?t=94188&highlight=long+starts

Sorry i forgot to say that mine is a 2000

Here's a brief synopsis:

The crank position sensor tells the engine computer the position of the crankshaft, which is needed to set timing. Because the ignition system is integrated with the fuel injection, and timing is set by the central computer, this is necessary. The engine will not start at all of the CPS is not working, because there is no default timing setting without it.

The cam position sensor, despite its location in the distributor, is not used by the ignition system, but to synchronize the fuel injection pulses. Depending on year and malfunction, you may or may not be able to start with this sensor dead, because the pulses are not critical to getting the engine to fire. However, a bad one in a 91-up system may prevent the computer from finishing its own startup sequence, and in consequence it will show a no-spark condition even though it isn't really a spark issue.

The throttle position sensor does just what it suggests: it tells the computer how far open the throttle is, and the computer adjusts fuel delivery accordingly. A glitchy one will usually give you odd idle speeds, intermittent stalling, or wandering rpms. I don't think any 4.0 can run with it completely dead or disconnected, but it's a variable resistor, so it doesn't usually die all at once.

The MAP (manifold absolute pressure) sensor is basically a vacuum sensor that compensates for altitude, throttle opening and other variables, and helps the computer adjust fuel mixture accordingly. If it misbehaves, you can get various symptoms, bad idle, stumbling, etc. I'm not sure whether the engine will start without it. Any 91-up XJ should send a code if it's electrically dead, but will not detect if it's just reading wrong.

As for your own problem, a new starter is unlikely. If it cranks at a decent speed, your starter is fine. That's all the starter does, after all. It does not influence anything else. I should modify that last thing: if a starter is very worn out, and drags, it is possible, though unlikely in this case, that it will pull enough current to lower battery voltage, and that may make the ignition less lively. I doubt this is the case here, especially since the XJ ignition is very robust and usually immune to these problems. That used to be an issue with older vehicles with non-electronic "points & condenser" ignition. However, if you suspect a bad starter, it might be worthwhile to check cranking voltage to make sure that you're not pulling battery voltage so low that the electronics don't work right. If there's a wiring problem at the battery, this might show up too. But this would not improve with the second attempt at starting.

I would still suspect fuel delivery. The pressurizing stage when you turn on the ignition is very short, only a few seconds. The pump will run and then stop. It does not keep priming. Just in case it's still a leakdown problem in the fuel pump, try this: turn on ignition for about 10 seconds, turn it off and immediately back on for 10 seconds, off and then immediately on and start. If this speeds things up, you have your culprit. If it doesn't, then it's likely something other than simple loss of fuel rail pressure.

definatly, you have a leaky injector or something, so your getting air until the pump catches up

Check your battery voltage and make sure the clamps are sparkling clean (both ends) The ground on the motor can make good contact sometimes then others it will be a little corroded (resistance shoots up as heat goes up) and cut power in half.

When you turn the ignition key on, you should hear the fuel pump run for approximately 3 sec. The pump has a built in check valve that helps maintain fuel rail pressure after engine shut down. Fuel pressure (early 4.0's) should be between 19 to 39PSI after setting for 30 minutes; if not, a bad fuel pressure regulator or a bad fuel pump check valve is indicated. Operating pressure should be 31 psi (engine running) with the vacuum hose connected to the regulator, and should rise to 39psi when the hose is disconnected.

Crankshaft Position Sensor (variously mentioned here as CPS or CKP) is intended to measure crankshaft speed - it's more properly called an "Engine Speed Sensor." There are notches all around the flywheel/flexplate, with three "index" notches corresponding to pairs of cylinders (#1/6, #2/5, #3/4.) This sensor will tell the ECU which pair of cylinders is approacing TDC.

Camshaft Position Sensor (properly CPS, usually SYNC here) is usually located in the distributor, and serves to tell the ECU which cylinder of the pair (the two approacing TDC) is coming up to be fired. RENIX can function without CPS input, OBD can not.

TPS - Throttle Position Sensor - is simply a potentiometer mechancially coupled to the throttle blade, which is used to tell the ECU just how open the throttle is. This is used to help determine engine load, can help to verify MAP signal, and contributes to fuel metering.

MAP - Manifold Absolute Pressure sensor - is little more than a vacuum sensor directly connected to intake vacuum. The MAP signal is cross-checked with the IAT (Intake Air Temperature signal) to determine the actual density of air in the intake stream - which is then looked up on an AFR (Air/Fuel Ratio) table in the ECU to determine injector pulse width for fuel metering.

I do concur with Matt Currie tho - my first suspect would be fuel delivery pressure and/or volume (but probably pressure - injectors are typically spec'd for a fairly narrow pressure range - too low, and they don't atomise the fuel well. Liquid gasoline does not actually burn...) Since you have a 2KXJ, you won't be able to access the regulator, since it's in-tank. However, you should still be able to test pressure and volume, with a proper test gage...

5-90

xjbubba said:

When you turn the ignition key on, you should hear the fuel pump run for approximately 3 sec. The pump has a built in check valve that helps maintain fuel rail pressure after engine shut down. Fuel pressure (early 4.0's) should be between 19 to 39PSI after setting for 30 minutes; if not, a bad fuel pressure regulator or a bad fuel pump check valve is indicated. Operating pressure should be 31 psi (engine running) with the vacuum hose connected to the regulator, and should rise to 39psi when the hose is disconnected.

Click to expand...

Because the 2k has its regulator inside the tank, you cannot test it separately. The running pressure for this one should be 49.2 PSI at idle, plus or minus 5 PSI. The 99 FSM also mentions that a drop to 0 PSI after cooling down is considered normal, due to gasoline contracting as it cools. But if the check valve is good, pressure should rise to spec within 1-2 seconds when the pump starts.

Thanks, ill let you guys know the outcome.

I think my 99XJ is doing the same thing. When it initially starts, it starts as if the battery is dead or dying. After the first "revolution" it fires right up.

I agree that fuel drain back is the likely problem, any bad sensors or even low Battery voltage will set the CEL light but a fuel delivery problem will not.

Be careful when reading these boards because the year being discussed makes a huge difference like the normally long crank times only applies to the 86-90 renix XJ's not anything newer.

I believe it was the fuel pump, for those with this same problem.

I changed out the pump two days ago, it was a real pain. Everything seems to be better, it still starts alittle longer than what I would like but nothing like before.

-Aaron