Cranks over but won't run

[Kevs GT]

I've got a 98 3.8 N/A it cranks over but won't run I've got fuel spark and compression I replaced the crank shaft position sensor it ran for 30 min but was misfiring turned it off and now won't run any ideas?? I'm thinking the ecu?? Thanx

[Scottydoggs]

check the wire harness for cracks or any bare wire showing, especially at the plug end, where the wires seal into the plug.

[bmx6454]

i had this same problem very recently haha.. turned out to be the cmp(camshaft position sensor) cheap and easy to replace.

[FordMan77]

Take 2 and walk away Other than that check for any wires you may have moved, as the belt runs near a few harnesses down there.

[bloaty57]

If not starting at all, I would've said crank sensor right off, but you said you changed that. And although it would run rough, the cam sensor shouldn't keep it from running.....hmmmmm ? The ignition module senses signals from the crank and cam sensors and controls the fuel pump so maybe there ?......I keep a used one that I know is good on hand just as a test piece, but the only other thing I could ponder was fuel pressure as you said you had fuel......? strange....

[bmx6454]

trust me, cmp will cause a no start... in my case it was telling the spark to fire at the wrong time, and the injectors to pulse at the wrong time. symptom that led me to this was it stalled at low speeds and in reverse a few times b4 it wouldn't start at all. does this ever happen to yours?

[bmx6454]

and for the record..... a bad cmp will always cause a no start sooner or later aslong as its going bad.....

[bloaty57]

bmx.......you might have a point with the 3.8's, not sure there..........they (GM) possibly incorporated something different in their CPU programming, but most of the systems they used on their 8 and 6 cylinders would allow the engines to start and run even if the cam sensor was totally shot. It would run rough, yes, but it would run. When the cam sensor was bad, or not sending a signal the cpu indexed it's timing origin for the engine off of the crank sensor. My brother is a fleet mechanic and is up on the latest computer tech stuff and he's corrected me on that point a couple of times..........at one point I said the same thing as you about the cmp. On my 5.7, just to prove it to me, he's disconnected the cps entirely and the engine still ran.

All that said though, I would gladly concede on the 3.8's because I know their crank sensors are a dual wave type, which is different than the standard GM singles on most of their other engines. One side is tach....the other is signal. Not sure if that would affect the cam sensor differently or not....

[bmx6454]

interesting point, but wouldn't a bad one act different than an unplugged one in some cases because the computer doesnt realize its bad and just going buy what the signal is saying? and no signal would make sense for it too just go off the last good reading.

[bloaty57]

If the sensor was bad, showing a signal stuck more in a high or low limit but not totally shot, it would run rough and not show a code as the computer see's it as working........in essence it is working, but not in normal parameters....i.e.; high to low, instead of high or low. If it's totally shot (or unplugged)......meaning no signal, the computer will show a code. I would surmise that maybe a sensor showing some code at all might run less rough than one that is totally shot, but that's completely an assumption. In either situation, code or no code, it should run rough enough to easily notice. In both situations, the sensors are bad.......just one showing a code and the other not.
As far as the computer running off of an old signal if the sensor was totally shot, no......it does a check of all systems on startup, which is why it throws codes when something is totally bad. If the CPU doesn't receive the proper signal from a sensor, it will throw a code and switch to default, "open loop", or LHM (limp home mode). Sensors that are totally gone pop up right away (no signal).....sensors that are borderline either won't, or might after a few miles. An engine should start, and run rough, but like I said, it's possible the 3.8's have a CPU function that doesn't allow it. I can't see why GM would change that process, but it doesn't mean they haven't.....so I conceded there.
When I bought my 00' GT it didn't run at all (which is why I got it for what I did .......but I knew it was okay mechanically. It wouldn't start at all but failed to show any crank sensor code. Even when we read the separate circuits with a scanner (not a reader), it would show a signal (the tach signal/wave only), so, not knowing it was a dual wave, we figured it was good. What we failed to realize was that it was a dual wave sensor (not a single)......tach on one side, signal on the other. We had tach, but no signal to start. I planned on replacing the electronics anyway with new stuff I had back at the shop, so I tossed on a new ignition module and a new cam sensor first......nothing. We dug deeper and found that the crank sensor operated with that 2 wave system so we replaced it and bingo.....fired right up.

I think Kev's problem is maybe two fold.........he replaced the crank sensor (assumably a new one), and it ran, but misfired, so.......assuming that there are no codes.....possibly a bad ignition module ?, bad coil(s)?.......TPS even ?......hard to tell. Best way in my opinion is a scanner.......it'll read everything, including the individual parameters of the sensors (i.e.; high to low, etc.)...........
I think even a horribly plugged converter could simulate it to a degree.......which is what I'm finding now. Anyway......hope he figures it out.

[bmx6454]

It didn't show a code for me and doesnt always. Mine acted up once, and then failed lol