what can this be...? please "diagnose"

[97tbird]

car : 97 Tbird 4.6L with 113k miles.

symptoms: rough, shaking idle when in D (and park, but more in D), engine sounds like weakly "revving" by itself at idle, and sounds like a diesel, especially after warmed up.
seems 95% fine when when driven, almost like normal, but I did feel less power than usual.
idles almost like normal when engine is cold.
Gas mileage has gone down a lot suddenly!

no check engine light.
plugs/wires are only about 25k old.
oil level fine. fuel filter replaced recently, and a/filter, too.
can't be bad gas, 'cause got some new gas when almost empty, and still did the rough idle thing.
o2 sensors are still oem.
no weird smells, etc.

recent codes:
within the last 2 years (twice): "bank 1 catalyst: efficiency below threshold"
(never did anything about it, 1st time had code erased, 2nd time went away by itself...the 2nd time was about 7-8 months ago, I think;
on both times when the CEL came on, car drove perfectly normal, that's why I never did anything about the code)....could this be it?

what do you guys think this could be...?
Thanks a lot.

[ August 08, 2006, 06:29 PM: Message edited by: 97tbird ]

 

[brianl703]

Only if the cat broke up and started clogging the exhaust..but then I think you'd hear it rattling around.

 

[97tbird]

would I hear that while driving, though?
thanks

 

[another Todd]

How about stuck open EGR valve if that engine has one?

 

[97tbird]

Todd. That's very interesting. I've never had trouble with the EGR valve itself, but about 3 yrs ago, the DPFE sensor (of the egr) went south, trigerring the code "EGR excessive flow" or something. had to replace the dpfe sensor (not the egr valve, though), and all was ok....the symptoms at that time was VERY similar to what I have now, only then it was rougher and it set a check engine light....
I'm hoping for a CEL, at least it'll tell me what to do.

 

[Clyde65]

Sometimes when a catalytic converter gets clogged up, they can cause excess heat to back up into the exaust and cause one or two exhaust valves to burn on their valve face causing a loss of compression in the corresponding cylinder - causing rough operation.

That happened to me on a 1982 Ford Granada, but it wasn't the CC, it was caused by a crushed exaust pipe by driving down the railroad tracks on new years eve of 1985 after having a few too many mugs of beer - 40 of them to be exact.

 

[punisher]

This is likely something that will need to be professionally diagnosed, unless you have an OBDII scanner you can review data with.

Several possibilities but the MAF sensor is my first suspect. Causes a lot of drivability problems, rough idle and lots of pinging/"knocking" during accel when warm due to excessively lean mixture. If you have data access, pull up the #1 and #2 short fuel trims, look for rich trims at idle and lean trims at cruise or during acceleration, that is the sign of a bad MAF. OF course you could always unplug the MAF sensor, cycle the key a few times to put the ECM in failure mode and take a drive. Make sure the "ck engine" light is on. If performance clears up, it is MAF time.

Catalyst codes- the sensors are never wrong on this one, seriously. If it says a catalyst is bad, not necessarily clogged, but bad then it is. A vacuum gauge is the quickest and easiest way to check for a clogged cat. Vac gauge= most underutilized tool in the shop.

I have seen coils go bad on those, fuel pressure regulators stick and the occasional O2 sensor take a dump. Never have seen an actual EGR valve stick on a 4.6 but I suppose it could happen. You will get the "excessive EGR flow" code before you realize a drivability concern.

 

[97tbird]

punisher,

Thanks for your comments/info.

I cleaned my MAF sensor a few weeks ago, and the idle and drive amoothness actually increased (then), so I know I didn't damage it or anything during cleaning.

...I'm sure the MAF is clean right now, which doesn't mean it isn't faulty for other reasons, like you said.

I'll try unplugging the MAF and driving.
Can you explain a bit more about cycling the key/ECM failure mode? how do I actually do it?

and, as far as you can see, could my cat problem cause these symptoms the car has? (apart from bad MAF?)

 

[punisher]

Unplug your MAF, cycle the key on and off a few times. This will usually get the ECM into failure mode, your ck engine light will be on. Just drive it until the ck engine light comes on and if drivability improves, you have your problem. T-Birds/Cougars were also prone to water ingestion. Not enough to hurt an engine, but enough moisture to ruin a MAF. Later models had a small drain under the filter box/inlet tube to let excess water out. This only applies if you have driven in heavy rain/high water recently.

 

[K-Man]

Use an OBD II scanner to look for stored "soft" codes. There might be something stored to give you some indication of the problem, and it simply hasn't triggered the CEL yet.