This truck belongs to a friend of mine. He told me he was driving down the road and suddenly the truck started running very rough and wouldn't stay running. Eventually it died while he was still on the road and it has not started ever since. It will crank and it is getting fuel but I'm under the suspicion that it is not getting spark or not getting it at the right time.

I scanned for code and came back with a p0340 (Cam sensor circuit fault) and a p0401. I knew about the p0401 from the past but the p0340 is a new one. I didn't have my multimeter with me to check the sensor but I did manage to get the top part of the assembly off. I didn't see any damage per say and the sensor part still appears to be in one piece. I plan on checking the wiring and connector for the sensor to make sure that's not a problem.

My question is would a bad cam sensor cause a no start condition? I know when the sensor goes bad it can cause the engine to run rough and stumble/chug/etc.,. but would it prevent it from starting at all? I'm going to test the sensor and get it back together so I can check for spark (which I should have done first) and we'll go from there.


(...Also, whoever picked that spot to put that sensor there needs to be hurt)

 

I have no first hand experience, but would expect a failed sensor could cause a no start condition.
The P0401 is a indicating a problem with the EGR circuit. You can search this forum (3.8/4.2) for P0401 and find lots of info.

I tend to be a firm believer in fixing what you know is wrong first, and then continue on if problems persist. So, given the situation, I'd say addressing the two OBD II codes would be the first order of business.

Keep us posted.

 

Worked on it yesterday and was able to verify a few things. The fuel pump is working as it should (43psi on KOEO and it holds pressure) and the wiring going to the cam position sensor seems to be good (got battery voltage, ground, and the signal wire circuit seems good according to the volt meter). I also scrutinized the cam synchronizer and checked to see if it was loose or had any play in it at all and it's not budging so I believe that part is good <crosses fingers>. I ran out of daylight and didn't get a chance to check for spark but from my understanding the ECM needs to see the cam position signal in order to control spark timing correctly and if the sensor is bad then there's a chance it will not spark anyways. I'm going to go ahead and get a replacement sensor for it tomorrow and we'll see what goes from there.

 

New cam position sensor is in and it still will not start. At this point I started pulling some plugs and could smell some fuel on them. I then proceeded to test for spark next. I checked the coil pack for any cracks and did not find any. I made sure the wiring that plugs into the coil pack was good. Finally I pulled the #1 plug and checked for spark while my friend cranked the truck and it is sparking.

I'm just about at a complete loss as to what is going on. We have fuel, we have spark, we have compression. What the truck is doing while it's cranking is it will try and backfire out the intake some with the gas pedal pushed down. It's acting almost as if the timing is off. I'm not even sure how to check the base timing on this motor and I doubt I could since it's computer controlled. If anybody has any ideas I'd welcome them.

 

FWIW Static Timing My distributer less '98 4.2 has degree marks on the crank pulley. So you could probably check static timing. like - number one piston at TDC should have the crank pulley timing mark at a certain spot and the cam sensor might have higher or lower resistance if the static timing is on. I'm not finding any process for this in my Haynes manual. I like to use a drinking straw in the number one spark plug hole.

An inductive timing light could help... without going out to look at my van I think this has you under the vehicle with the light while the motor is being cranked. I guess with the FI harness unplugged it would not be too much of an invitation for Darwin to visit. I'd do it, but I also shop vac'ed out my gas tank last year so...

 

I know I'm a little late on updating this thread but here it is.

He ended up having the truck taken to a shop and within a couple of minutes they found that the harmonic balancer had spun on itself (aka the rubber was split all the way around.) It didn't dawn on me to check that when I was working on it but it seems that the bad balancer was throwing the ignition timing off which would explain a lot. New balancer and a new fuel pump since the old one died over the past few months and she fired right up.

 

harmonic balancer slip and threw off the timing? Is that because it wouldnt line up correctly with crank sensor ? i am gasping here ,i think i have done everything i can. And ur post sounds like it might be my problem. I appreciate ur input more of why the harmonic balancer was the problem .

I have 98 4.2 had the infomous sdquel for about a year, got the code 2 weeks ago noticed a little stumble at take off . The engine light already on from 02 sensor . Truck dies backing out of the driveway, replaced the the sycronizer assembly , sounds the same , just cranks over. also tried the voltage methed , and have spent many hours searching threads

Thanks