Truck died and won't restart - the diagnosis begin

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rationull

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Originally Posted By: wtd
You don't have to pull the upper intake to replace cap + rotor. Just lay on top of intake and you can get to them. I would replace the plugs. A lot of people say this engine runs better with a spark plug gap of .045 which is what I run on my 98 7.4L I have also replaced my wires and coil. You can do the same to rule those out as problems. Wayne
I don't know -- with the way the wiring harness is set up it's pretty tight behind the intake. I'm sure it can happen but it seems like it's be a PITA. Of course it also would've saved 1.5 hours taking everything off! I took it apart today. Nothing is visibly broken in the cap/rotor but they do look fairly worn. Given what I saw with my timing light yesterday I feel like I should replace the camshaft position sensor too since it's right in the distributor and pretty easy to access. I considered the ignition module too (on the same bracket as the coil) but it's pretty expensive and I have no reason to believe it would fail this way. So my current list is: Definitely replace: Cap, rotor, cam position sensor, coil Probably replace: Plugs and wires (although I could technically replace those later). I may get lazy on the plugs and wires for now since it's doubtful they're actually the cause of the problem (not all at once, anyway). It will be another week or two until I get the chance to actually replace the parts though, depending on whether some family visits happen.
 

wtd

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The 7.4L seems to use up cap & rotors pretty quickly, especially if they are aftermarket. I recommend Delco replacement parts on those.
 

rationull

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OK, it's been a long time, but I just finished putting everything back together today. I replaced: cap, rotor, cam position sensor, plugs, wires. I bought a coil but didn't have much luck drilling out the rivets holding the old one on (I really need to get a 120V AC drill and some new bits) and it was getting late in the day so I just put the old one back on to see what would happen figuring I can either replace it later or maybe just return the new one. The engine started right up. It's got a CEL with code P1345, which hopefully just means the timing is a tad off. I ended up having to remove the distributor because I broke off a screw head when putting the new cam position sensor in and while I was pretty careful labeling its position I'm guessing I was a degree off or something. At least I hope it's that instead of a problem with the new sensor. One of the plug wire connections on the dist is pushing up against the upper intake manifold too, which gave me a hint that the dist position might be a bit off. So the verdict is that the problem was spark, not fuel. Since I shotgunned all this stuff (thanks for putting the distributor behind the intake manifold, GM!) we'll never know what specific part was causing the problem, but from the conditions of the old parts I'd guess it was the cap, although the CMP sensor seems likely too even though it wouldn't show any wear visibly. Given the engine starts and runs now, is there any reason for me to replace the coil vs just return the new once, since removing the old one is turning out to be a pain?
 
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I was just about going to ask you about an update thumbsup. It was all pretty confusing. I thought you had mostly normal spark (and abnormal fuel pressure) but when it would not start even with starting fluid, it kind of eliminated the fuel and pointed to spark problem. I was guessing you a badly worn cap and rotor and maybe an ignition module problem but I guess there was no module problem. My guess also is the distributor postion is off and is what's setting the code. I would just hold off on the coil and return it. There doesn't seem anything is wrong with the coil, and may not need one for who knows how long. I don't like to replace parts unless I know they're bad.
 

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I was pretty convinced it was spark after hooking the timing light up to ALL cylinders -- after just checking the front two it seemed like spark was fine, and it wasn't until I saw the one firing faster than the others that it was clear. It does seem strange though, this is not what I would've expected from a cap/rotor. I do wonder if the cam sensor was actually the cause but it seems like that would've set off a CEL instead of just dying. Now, I don't have a scanner that can read the timing on this thing (special GM CMP Retard PID) but I have a hunch I know in which direction the timing is off. If I can get my distributor wrench down there or find some other way to loosen the distributor without taking anything apart (maybe buy a set of flex head ratcheting end wrenches?) then I can try adjusting it a little and see if the code goes away.
 

rationull

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Does anyone with experience on these engines have any helpful tricks for loosening and retightening the distributor clamp hold-down bolt while the thing is assembled? It's really tight, between the intake manifold, the plug wires, and the wiring harness and near as I can tell the only way to get to it is to have a long flex head end wrench or maybe a really low profile flex head ratchet -- and even then I'm not sure. I might be able to get a half decent picture if necessary. I know there's got to be a way because there's no way a shop is going to take the manifold off just to adjust the timing. Figured if anyone has any tips now's the time to ask, before I end up buying tools that don't quite fit.
 

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OK, to close this up for real, in case anyone else as mechanical challenged as I am finds this thread in a search. In order to loosen and rotate the distributor without disassembling much, I ended up just taking off the intake "resonator" (or whatever it is .. the big empty box with "Vortec" on it) and unplugged the #8 and #2 plug wires from the distributor. That gave me enough room to get the distributor wrench onto the clamp hold-down bolt and with a 3 inch extension on that and a flex head ratchet, I could turn it about 1/8 turn at a time. It took only took about 3/4 of a turn to get it loose enough to rotate I think. I repositioned the distributor to something that looked more "reasonable" with respect to the position of the plug wires relative to the upper intake on the driver side. Then I cleared the codes and started it up. Idled it for a while, revved it, held revs at ~2500 for a while, no CEL. So I think it's good to go. Haven't gone for a drive yet, want to make sure I have time to deal with getting towed again before I take it out. And I ended up just returning the coil.
 
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