98 Cranks no start

I'm trying to figure out why my 98 will crank, but not start. It did this one before about 2 years ago and on the advice of the local AAMCO guy, I tried replacing the cam position sensor and the problem went away. Its back now, and I have no idea why. If I leave the truck parked with the engine slightly uphill, it will not start. It cranks just fine, but won't start. Other positions it sometimes still won't start. The odd thing is it will pop start very easy every time it refuses to start with the starter. That makes me think its a sensor going bad, and pop starting makes the computer think the engine was already running. Also, I believe it is running rich as the intake is full of carbon and the fuel mileage is bad right now. That may be unrelated, though.

What I have tried so far:
1. Spray fluid in the intake - no change. Still won't start
2. Fuel pressure seems good
3. Replaced water sensor. It was out of spec. Made the problem worse.
4. Checked CAM sensor with O-Scope. Got a nice decaying pulse at about 500Hz with the engine running at about 1000 RPM. Seems right.
5. Checked crank position sensor with O-Scope. Got a ~1000Hz sine wave with the engine running at 1000 RPM. Again, seems right.

I am now completely at a loss as to why it will not start sometimes, and is hard to start most of the time. Sensors seem to check out. I would expect if they were failing, I wouldn't get a good signal. There is a code in the computer, but it is P405 for evap system. I just had the rear end replaced and did some work back there, so probably knocked something loose. Does anyone have any idea where to check next?

Could be battery voltage when using starter motor

Good battery will read above 12.3volts with key off

A 2 to 2.5 volt drop is expected when starter motor is used, so test battery voltage when cranking, needs to be close to 10volts, if it is dropping below 10v too much the spark will be very very weak, so no start
Bump start of course has full 12volts for starting/spark

Voltage could be dropping to much because of dirty battery cables, at battery or at starter motor AND engine GROUND cable, so check those
Starter motor could be going bad, drawing too many AMPs
And of course battery itself
New battery will read 12.8volts +
5 year old battery about 12.4volts
6-7 year old will be getting down to 12.2v and time to find a battery sale
As temperatures go down so does battery power, battery uses a chemical reaction to release AMPs, lower temps slow down that release, so a battery at 70degF might work fine, but at 50degF it may not
Beginning of Winter is when more battery's are sold, lol.


Good time to test charging system as well
After startup battery voltage should be above 14volts but below 15volts
After engine has run more than 5 minutes, and idling, battery voltage should be approx. 13.6volts, under 14volts, above 13.2volts
If voltage is still above 14volts then voltage regulator is bad and battery is getting "cooked"

With engine still idling, turn on all the lights and heater fan to High
Voltage should drop then come back up to 13.6v
All is good

If voltage drops and doesn't come back up, REV engine and see if voltage comes up, if so then alternator has 1 failed field, there are 3 fields so as long as you don't idle for long periods with lights and fan on there should be no problems, but if you come across an alternator sale or a used alternator, pick it up

RonD,
Thanks for the reply. I don't think its a weal battery. It cranks fast like it always has. I did check the voltage with one clip on the chassis ground and the other on the battery clamp bolt. 12.6V with the key on. Dipped to 12.3 when cranking, and right back up to 14+ when the engine started. I am going to take it apart and clean it anyway. I have added a jumper port to the truck, and the new clamps do look a bit dirty, but I don't think its my problem in this case.

I think you mean 10.3v
12.3v during cranking would be a bad reading, starter motor pulls 70-100amps, so voltage will drop quite a bit


Since it doesn't start using Starter Fluid(ether) then Spark is what is not working, for sure, so don't waste time on fuel issues.
Ether has a very low ignition point so even weak spark should ignite it, which is why it is a good starter fluid for gas or diesel engines
Check connection on coil pack, although bump starting should also be effected if that the problem.

Reason I suspected battery voltage is because that's the only difference between cranking and bump starting, all the same sensors and controls are used for both methods, difference is starter motor is active or it isn't.

crank sensor.
Mine does this.
Since I bought a crank sensor and put it in the glove box, it starts every time.