Thread: Bad cam/crank sensor?

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Hi guys,

I have a 94 Thunderbird 4.6L V8, and I have a few issues. First of all, say I start the car, drive it for 20 mins, kill it, and go restart it 30 mins later, she won't start. If she gets cold, she will start again. Also, while I'm driving it, if i push the gas pedal gradually, it will sputter(rpm goes up and down by 200) and eventually she will get up to speed. I have replaced both coil packs, plugs, wires and still a problem. Now, under heavy acceleration, she even backfires. To me it feels like a missfire, like a timing issue or something. No diagnostic trouble codes were set. I heard that the 4.6 is notorius for crank sensors, however, if i'm doing one, mind as well do the other too(cam sensor). Any ideas guys?

Would you hint towards theTPS even though it sometimes surges at idle too?

I tested everything, besides the cam crank sensor. Ok, what happens is, when it won't start(cranks over, and it won't fire), its got gas pressure, no vacuum leaks, cylinder compression i good(at least thats what my co-workers say). Now, the thing it doesn't have is spark. After it cools down, it fires right up. At my shop I've had a car that would die while driving and wouldn't start until it has cooled down. The answer was a crank sensor. I'm going to take my chances tomorrow, and see what happens after i change the sensors on the Thunderchicken(common sense to change them every once in a while). I'll keep you posted. Thanks for the hint, i'll check that first thing.

I did it the old school way. Grabbed a prybar, and while the car was running, i tapped the crank sensor, and yes it stalled. I changed the sensor, and now there still is a minor missfire, but, i ordered a TPS and i will put the cam sensor in also. Do you know where that TPS is located? On the throttle body?

ussually opposite side of the throttle cables, the shaft goes into it... on some cars you may have to adjust the sensor position.....

OK, I replaced the crank sensor, TPS, cam sensor, and the missfire is gone, BUT...something I have never noticed before, i believe the catalytic converter is clogged or gone. My exhaust sys is like this:1 cat per each pipe coming out of the manifolds, then there is a joiner catalytic converter, a resonator and then my pipes(no more mufflers). Now, the cat that is from the bank of cylinders #1,2,3,4 rattles when you bang on it with your fist. The joiner cat rattles too. I believe that would be a part of my hesitation or stumble upon my acceleration too. I can hear it in the exhaust pipes when u push it hard. I believe I have old broken parts in the cats that block the exhaust gasses from passing freely. Any ideas?

I know you might say its the heat shields, but no, its not the heat shields fault. They still have the good original welds.If I'm gonna punch them through I'm going to have no back pressure

well.... if you hooked up to a smog machine and gave me some readings i could tell you if the cats are any good, but thats about the only way to tell.... according to the state of california a cat is considered damaged the second exhaust passes thru it

What readings do you guys have in California? Over here in Canada we have readings for NOx, CO, HC. I just ran my car on the dyno for an emission test(i'm licensed), and it did alright. I think that for NOx the limit was 400 and i had like 215 or something. That was the only bad readings. The others were just fine and dandy.

high NOx is not bad cats, bad cats will you give you high hydrocarbon levels with the CO high as well.... high nox alone ussually is a dirty EGR or MAF sensor thats not doing what it should cause its all dirtied up.......but can also be caused by sensors in open loop we have the same readings here in california except we're much more strict on a vehicle when it comes in for emissions testing..... i'm a licensed smog tech here but i just left the automotive business to work in real estate doing mortgages....... theres a lot of that money out there right now so i gotta get it while i can

Yeah, I hear ya, i feel the same, but, I'm still young, and I love working on cars, so I wanna carry on with it for a bit. Ok, I checked the readings on the sheet again, and it turns out its not NOx, its HC. So that would point out a bad cat. I'm gonna call my piping/cat place tomorrow, because apparently they have updates for the exhaust, and instead of 3 cats, it's going to be only one, with slots for the O2 sensors.

nice..... how strict are smog laws in canada? i know here in california we have the most strict out of the entire US.... its really difficult to get an older car to pass.... luckily we dont have to smog anything 74 or older though.... i know none of my cars would pass

Well, the limits are pretty tight, for ehample my 91 Caprice wouldn't pass, so i just put it on the dyno, and adjusted the timing, pulled out the EGR, and eventually she passed. I know, "cheating is wrong". Cars that are exempt from E-tests are cars that are 20 years old or more, so for example, cars that were born in 86, are exempt from the test. I want to buy a 83 Cutlass Supreme after I'm done with the thunderchicken.

sounds similar to our system on strictness.... we were supposed to goto the 20 years thing like 10 years ago, but you know how liberal tree hugger commies are.....

Yeah, I hear you buddy. It is a pretty stupid thing to do with these emission tests, because the real issues are the big diesel rigs, busses and others. Those are bad for the environment, not our little engines. Just another way for the goverment to make money