Blogs Car Info Our Show Deals Mechanics Files Vehicle Donation

Cam Sensor Red Herring?

I drive a 97 Ford Thunderbird 4.6L V8 with 135K miles. I’ve got a check engine light that gives a cam sensor code. The car also idles rough, and recently stalled on me when I went to pull away from a stop.

The mechanic put a new cam sensor in it, only to have the check engine light return (immediately–same code), and it still idles rough. I’m also pretty sure that I’ve tried replacing the cam sensor before, in the last 6 months or so.

Before I put myself at the mercy of the Ford dealer (as my mechanic recommended), I’d like to have some idea what I might be dealing with. When is a cam sensor problem not a cam sensor problem?

When the wiring that connects it to the computer is the problem. What exactly was teh code? Post the number in the format P0xxx and we can give you more information.

Obtain a copy of the 13 page Ford TSB (Technical Service Bulletin) # 02-22-1

It applies to several Ford/Mercury models and model-years, including "FORD: 1994-1997 THUNDERBIRD

Incorrectly installed gear driven camshaft position
(CMP) sensor synchronizer assemblies may be hard
to diagnose. Vehicle may exhibit poor fuel economy,
driveability Diagnostic Trouble Codes (DTCs)
P1336, P1309, P0340 with MIL light on. Loss of
power, surge, hesitation and runs rough on
acceleration may also be present.”

P1309 = Misfire Detection Monitor not enabled
P1336 = ?
P0340 = Camshaft Position Sensor Circuit Malfunction

See if that’s it.


@keith - not sure the code. If it’s not on the service record, I’ll have to ask.
@CSA - that’s definitely interesting. Any good ideas on how to bring it up with my mechanic without saying “Maybe you just did it wrong?”

My Computer And My Computer Savvy Aren’t Up To Linking This Bulletin From My Source. Perhaps Somebody Can Search And Post A Link To A Download For It.


There’s also at least a decent chance this rough idle is not related to cam sensors at all. I can’t be specific as there are too many loose ends but just wanted to point that out.

Does the car seem to run and drive out fine going down the road?

Ask if you have a worn timing chain to make the cam position sensor indicate late cam timing.

Connecting a vacuum gauge might help. It’s cheap, easy to do, and any anomaly that exists should steer the diagnostics in the right direction.

@keith - it’s P0340.

@ok4450 - it runs fine going down the road. It’s only rough when idling in Drive, like at stoplights. Pop it into Neutral, purrs like a kitten. Put your foot on the gas and pull away, it runs nice and smooth.

While it’s possible a cam sensor could do this I suppose, if the car were mine I would be looking at a vacuum problem, Idle Air Valve problem, or a slightly lowered engine idle speed. Those things are kind of mingled and that’s why my suggestion about a vacuum gauge.

Hm. I had an issue once before with a dirty idle air valve, but the problem I had then was that the car wouldn’t start. If you cranked it with your foot on the gas, the engine would turn over, but when you took your foot off the gas, it would die. I was told at the time that the idle air valve is only used when the engine is cold–which made sense, because if you sat with your foot on the gas and revved the engine long enough, then it would run fine.

I took it to 3 different mechanics before one of them could identify the problem.

What I’m seeing now is that it starts fine, runs rough when it idles, but it seems to be worse when the engine is warm. When it’s stone cold, it’s quieter.

But that IS interesting. I hadn’t considered an air problem. I wouldn’t have guessed that there was a close enough link that it would register as a cam sensor issue.

OK, the latest read is now giving P0171 and P0174. They want to replace the computer and O2 sensors.

Your engine is runnig lean on both banks. If your mass air flow sensor is dirty is isn’t measuring the air that enters your intake manifold properly. Try cleaning the mass air flow sensor first.

Yeah, and if that doesn’t fix the P0171 and P0174, check these next:
Fuel pressure (I’ve seen PLENTY of lean codes because of fuel pressure issues)
Check for vacuuum leaks (that includes exhaust leaks in front of the upstream oxygen sensors)

I’d be very skeptical about any advice those guys are giving you at this point. Replacing the PCM and the 02 sensors is only worth considering if everything else has been ruled out (granted, there are some situations where a TSB does instruct you to replace the PCM and the 02 sensors, but those aren’t common scenarios). Did those guys say why they want to replace that stuff?