: BCM causing rough idle?



hankk17
03-06-04, 02:54 PM
Hey guys. I'm still chasing my rough idle on the 99 Eldo.

So far I've:

1) replaced the Fuel Pressure Regulator (the old one was seemingly OK but I replaced it anyway)
2) run 2 bottles of top engine cleaner through the engine
3) cleaned the throttle body so it was spotless and sparkling
4) unhooked the negative lead of the battery to reset the throttle sensor each time I've cleaned the throttle or run top engine cleaner through.

At this point, the rough idle persists and has not changed. The RPMs bounce all over and the car surges a bit and it very nearly stalls.

I have checked the computer and it is throwing two codes consistently - both of which pertain to the BCM being faulty/screwed.

My question now is - can a bad BCM cause the rough idling I'm seeing or is the BCM separate from the engine entirely?

My next question - I've located a few places online that will sell me a used BCM for this year Eldo. Can I simply replace my faulty BCM with a second hand BCM and then go on my merry way (ie, push-pull-click-click?)?

My last question is - If all of the above regarding the BCM is just a red herring, what's my next step? Crankshaft Position Sensors?

sorry to repost this, but I'm seriously stumped.

Thanks!

-Hankk17

El Dobro
03-06-04, 03:14 PM
The item that controls the engine is a PCM (powertrain control module). The BCM is the body control module and usually covers things like lighting, heating, etc.
If your PCM is bad, a replacement would have to be programmed by the dealer. You can't just pop another one in. As a matter of fact, just about every module in the cars nowadays has to be programmed to the car.

JimD
03-06-04, 04:07 PM
Hey guys. I'm still chasing my rough idle on the 99 Eldo.

At this point, the rough idle persists and has not changed. The RPMs bounce all over and the car surges a bit and it very nearly stalls.

I have checked the computer and it is throwing two codes consistently - both of which pertain to the BCM being faulty/screwed.

-Hankk17

What codes are you getting?

BeelzeBob
03-06-04, 11:18 PM
Hey guys. I'm still chasing my rough idle on the 99 Eldo.

So far I've:

1) replaced the Fuel Pressure Regulator (the old one was seemingly OK but I replaced it anyway)
2) run 2 bottles of top engine cleaner through the engine
3) cleaned the throttle body so it was spotless and sparkling
4) unhooked the negative lead of the battery to reset the throttle sensor each time I've cleaned the throttle or run top engine cleaner through.

At this point, the rough idle persists and has not changed. The RPMs bounce all over and the car surges a bit and it very nearly stalls.

I have checked the computer and it is throwing two codes consistently - both of which pertain to the BCM being faulty/screwed.

My question now is - can a bad BCM cause the rough idling I'm seeing or is the BCM separate from the engine entirely?

My next question - I've located a few places online that will sell me a used BCM for this year Eldo. Can I simply replace my faulty BCM with a second hand BCM and then go on my merry way (ie, push-pull-click-click?)?

My last question is - If all of the above regarding the BCM is just a red herring, what's my next step? Crankshaft Position Sensors?

sorry to repost this, but I'm seriously stumped.

Thanks!

-Hankk17

I would doubt that an idle problem like you describe would be caused by the BCM...

The odd thing is that the 99 has a very sophisticated and robust engine misfire diagnostic that should be turning on the SES if there is really an engine miss.

I would tend to think that maybe the crank sensors would be a possible good guess at this point as possibly there is an intermittant on one of them that is causing an occasional idle glitch that is brief enough so as to not set the trouble code for a crank sensor....

kevinK
03-06-04, 11:19 PM
Could it be the IAC? Just a guess.

hankk17
03-09-04, 09:56 AM
Thanks for everyone's input on this!


Here are my codes as of this morning:

P0741
B1552
C1255
B0533
B1552 (it came up again)
B1558

The "service engine soon" light has come on and stayed on this time..

richscho
04-08-11, 11:35 AM
... A few suggestions for things that I have found to solve such problems in the past.

BTW: The BCM is NOT likely the problem, as has been mentioned above.

Idle problems are often related to air/pressure problems in the intake housing.
- Clean the PCV valve - whether you think it needs it or not. (It's all metal, so soaking in a solvent will not harm it.)
- Check on the MAP sensor probably the most likely culprit. (Inconsistent pressure regulation is "confusing" to fuel injectors.)
- Check any vacuum lines running out of the intake system. (Generally related to tranny issues also, but who knows...?)

Trouble codes are great as far as they go, but sometimes they tell you about other systems that have been influenced by something else that is the 'real' problem.

If the BCM is seeing problems, the only connection to the engine idle must be related to hiccups in signals sent 'to' the BCM - Or... You have an entirely separate array of problems. The idle is not being directly affected by the BCM!!! They don't even "know" one another...

My guess: You need to replace the MAP sensor. (Assuming that the PCV valve has been cleaned at least once over the life of the vehicle. Note: the thing that lies directly beneath the MAP sensor - and therefore lies between that sensor and the intake housing environment - is the PCV valve.)

It's a pretty simple (and inexpensive) repair attempt, to clean the PCV valve, and (using some PLASTIC-SAFE solvent) attempt to rinse any deposits out of the MAP sensor. Be gentle -- or you will need to replace the MAP sensor anyway.

You are simply trying to remove any gunk that might be making the pressure transducer (within the MAP sensor), less sensitive.

Submariner409
04-08-11, 11:55 AM
Wow !! How did you come across a 7 year old thread ?

These cars do not have transmission vacuum modulators - so no vacuum line there.

BCM is one of the Body Control Modules - PCM is the Powertrain Control Module, and it does, in fact, control idle speed and mixture through signals to either the ISC (Idle Speed Control) motor or the IAC (Idle Air Control) solenoid, depending on year and model.

The PCV valve in a 99 is in the passenger end of the rear cam cover. The MAP (Manifold Absolute pressure) sensor is stuck into the top of the throttlebody. That PCV valve is a rattle-type and can be replaced for about $2.50. (Later engines use a fixed orifice - no PCV valve as such.)

There are tutorials on IAC, ISC, MAP, and TB cleaning down in Discussions, Item Specific: Cadillac Tech Tips.