Acting like it's limited at 3000RPM [Archive] - GrandAmGT.com Forum

PDA

View Full Version : Acting like it's limited at 3000RPM


pauljs75
10-02-2013, 03:32 PM
I've got a funny problem with my '01 GT Sedan with LA1.

I haven't been doing much highway driving, but now that I did today I stumbled on something. Still has good power on the low end, no SES light, nothing funny coming out of the exhaust, air filter looks ok, fluids look ok, spark plug wires look ok...

But somewhere over 3000 RPM, the engine drops RPMs like it's hitting a rev limiter. (And I know the limiter on these cars is somewhere upward of 6000RPM.) Nor is it the kind of lugging behavior you get where an engine wants to die or stall out. However the not wanting to rev any faster is definitely a problem when you have to merge or pass at highway speeds.

Doesn't seem to be the transmission either, checked it after work while parked and it does the same thing.

So what's likely to be doing it? Think it could be knocking/pinging from a batch of bad gas, or what? (The car has been a good runner and reliable, so I'm likely to fix it whatever it is.)

rjs_boy
10-02-2013, 07:03 PM
I would check air pathways. Intake to Exhaust. If it is always around a certain RPM that you hit a wall, I would look for something that would cause that.

Can you rev it any higher in neutral?

bricooper78
10-02-2013, 07:12 PM
can you get a vid clip?

sugar ray
10-02-2013, 10:12 PM
My 02 did the same thing once. Your symptoms described point to a clogged cat. Converter. I'd start there. Chances are you need a new one.

pauljs75
10-03-2013, 09:29 AM
I'll consider that. Knowing my luck, that's probably what it is. However I thought OBD-II had sensors both before and after the cat, and that a bad catalytic converter would trigger an SES because the second O2 sensor would read inconsistently. Guess not, huh?

And like I said, air filter looked ok. Don't think it would be airflow unless some mouse or something really got up into the ram-air duct trying to start a nest or something. But I don't see any signs that would indicate that. (And I drive the car often enough where it shouldn't be too comfy for a critter to camp out in.)

rjs_boy
10-03-2013, 05:17 PM
Depends on how much air is getting to the 2nd O2 sensor...how well that sensor is working...etc.

The air getting through the converter might be within acceptable ranges to not throw a code...or the sensor could be giving bad readouts...or both.

Will the engine rev over 3000 in neutral/park?

pauljs75
10-07-2013, 05:04 PM
Hmmm... I thought the rev limit was lowered from park or neutral anyways. (Stock programming on ECU, it should be iirc. About 3500 without load. Somebody here should be able to confirm this with a car that isn't acting up.)

The shop I go to is a dealership, but I think it's worth a little extra since they do have fairly good guys. (They don't seem to do some of the half-assed stuff of the local shops I've dealt with before. There might be a good local place that's more affordable, but I don't have the time to try finding it.)

Apparently they did the exhaust backpressure test, as well as ignition and fuel pressure checks. But couldn't figure it out. By the book those looked sat. And the mechanic also saw the problem for himself, so he knows it's just not what I said.

Yet I think the lower rev limit in park or neutral by default may be a good clue. Where my car is hitting when it acts up seems about where that is. Imagine it hitting the park or neutral limiter while under load instead of free-revving.

There is one thing I could think of that's left. The gear selector switch might be giving a wrong value. (Bad connection or worn out contacts, etc.) I mentioned it to the service write-up guy. If the computer sees a low gear or neutral while the car is in drive, won't it kick in the lower value over-rev limiter to prevent damage?

I don't have the diagnostic computer to plug in to the OBD port to get that data, but I'm fairly sure the dealership guys would have that and be able to see what info the computer is seeing. Row through the gears and see if the OBD readout matches the selector on the PRNDL.

rjs_boy
10-07-2013, 07:59 PM
Rev Limiter in Park should be around 4000, you should be able to hit that easily and watch it bounce around 500rpm as it hits it. I wouldn't sit there and make it hit the limit for any prolonged period...but it shouldn't hesitate to hit the limit, fall to about 3500ish and then get right back to the limit.

If you have an android phone, I highly recommend investing in a bluetooth OBDII reader and the TorquePro app. For less than $20, you'll have a code scanner and data logger that will help find quite a few potential problems.

Coolant Temp
Transmission Temp
Torque Converter Slip
MAP Pressure Reading
Ignition Timing Advance / Knock Retard
TPS Position
Intake Air Temp
O2 Sensor Readings
Fuel Trims
MAF Flow Reading
+more


OBDII Bluetooth :
http://www.ebay.com/itm/ws/eBayISAPI.dll?ViewItem&item=310678002251

TorquePro App for Android:
https://play.google.com/store/apps/details?id=org.prowl.torque&hl=en

bricooper78
10-07-2013, 08:12 PM
The tranny is shifting properly tho, right?

If you floor it, will it kick down into passing gear?

pauljs75
10-08-2013, 07:49 PM
Anyhow it wasn't that... (See title.) But with funny RPMs, transmission likely had trouble getting the right shift point anyways. At least when going hard enough to trigger the problem. But it did work ok when going easy on it to get up to speed.

No related codes thrown with SES, but apparently guy finally found it's the MAF sensor. Considering the resulting behavior looks a lot like hitting a limiter (up to an RPM and cutting out briefly so it hovers below that RPM), I'm surprised no SES was triggered by the fault.

I'm curious how the guy found it. I guess it took re-doing some tests with the OBD scan tool plugged in and logging? (Just how far off do readings have to go for the idiot light to come on?) Another funny thing is that two weeks prior to finding my car with this problem is it passed emissions with no problems.

Hopefully it'll be driving like a champ after getting out of the shop. It hasn't been bad for a vehicle going past 10 years on relatively minimal maintenance. (Just the regular fluid changes as needed.)

So yeah, all other things excluded (they looked good) and no engine light... Put that MAF sensor higher on the check list. :hammer:

bricooper78
10-08-2013, 08:44 PM
Hmm. Well, at least it's on the mend, that's good at least.

rjs_boy
10-08-2013, 10:38 PM
would make sense though...if the MAF was only reporting so much air going in, but still giving a valid reading, the computer wouldn't really know much else. You'd think that the TPS and fuel trims would've been affected at the least though.

Good to hear your back up and running!