'99 MGM Problems on Problems! P0420 and P0430

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DesertRyder
Posts: 3
Joined: Wed Apr 19, 2017 5:09 pm
Vehicle: 1999 Mercury Grand Marquis

'99 MGM Problems on Problems! P0420 and P0430

Postby DesertRyder » Wed Apr 19, 2017 5:22 pm

Long story, I'll try and keep it short.

When the car was first purchased I had only 2 codes - P0420 and P0430 - catalyst system low efficiency on both banks. No problem, cats aren't that good, it can wait. Fast forward another month and I'm pulling into a gas station when suddenly the car begins to chug and jerk, as if it's perpetually trying to stall. This jerking is usually accompanied by a diesel-sounding rattle coming from under the car, especially when accelerating up a small hill. Additionally, if the car is put in reverse, it immediately stalls. However, if I give it just the right amount of throttle as I drop it into "R", it will stay running, but the vehicle will perpetually jerk back against my brakes as it attempts to stall out - much worse than it does when in Drive.

Plugged a code reader in again and got a code saying my system was too LEAN on both banks. Pardon my ignorance, I don't remember the codes for these issues. So far I have replaced my MAF and I am fairly certain my TPS and IAC aren't the issue.

I stopped by the local automatic tranny rebuild shop and the mechanic suggested it could also be clogged cats that are causing the jerking/stalls under load. Nope. Cut the 2nd bank of catalytic converters off and all the exhaust behind it - still stalls. And now it's loud.

Any ideas? I believe I've ruled out the exhaust being an issue. Possible transmission troubles such as a torque converter? Or maybe a serious fuel trim issue? I'm at a loss for what to do here without a $3000 scan tool to tell me what the problem is.


kev2
Posts: 6447
Joined: Sat Aug 29, 2009 4:24 pm

Re: '99 MGM Problems on Problems!

Postby kev2 » Wed Apr 19, 2017 5:50 pm

mercury grand marquis MGM :) - learn something new every day...

Ok we will need to replace those cats.
Lean codes are as you said a fuel trim issue - first look for a vacuum leak, also a fuel pressure check, Getting those codes to us could help, just the codes will not require a 3k$ scanner.

Might use the old school carb spray trick to locate a vacuum leak - youtube has a tutorial on this easy cheap old school trick.
A fuel pressure gauge is often a loaner or inexpensive rental if you do not have one.

DesertRyder
Posts: 3
Joined: Wed Apr 19, 2017 5:09 pm
Vehicle: 1999 Mercury Grand Marquis

Re: '99 MGM Problems on Problems! P0420 and P0430

Postby DesertRyder » Thu Apr 20, 2017 9:48 am

Just plugged my code reader in this morning - 2nd bank of cats and exhaust gone. Only getting TWO codes now on my jerking/stalling vehicle. P0171 and P0174 - my code reader simply says System Too Lean Bank 1/Bank 2. Which is funny since my Haynes Repair Manual for this vehicle is telling me it's something to do with heated o2 sensors switching slowly or not at all.

Starting to wonder if this IS a tranny issue and I'm just not seeing it since I don't have access to a tool that can read transmission codes.

Do these symptoms really seem like a vacuum leak? I sprayed some brake cleaner around my engine compartment a few days ago and didn't notice any surges in idle. So if it's a vac leak it's a sneaky one.

kev2
Posts: 6447
Joined: Sat Aug 29, 2009 4:24 pm

Re: '99 MGM Problems on Problems! P0420 and P0430

Postby kev2 » Thu Apr 20, 2017 10:28 am

yes there is often differences in code descriptions, why we always ask for 'details'. But
A ford being lean is so common - as mentioned do the vac leak checks.

Ford wll set most trans codes in the P0700 range so most readers will indicate those.
The lean condition could also be fuel pressure, or bad sensor readings, even an EGR could cause the lean. With your exhaust system as is the readings will be skewed - no cat or piping.

Is there any trouble when reving eng in N or park?
Any difference hot or cold?
Have you checked trans fluid ?

https://www.obd-codes.com/p0171 P0174 is the same other cylinder bank

DesertRyder
Posts: 3
Joined: Wed Apr 19, 2017 5:09 pm
Vehicle: 1999 Mercury Grand Marquis

Re: '99 MGM Problems on Problems! P0420 and P0430

Postby DesertRyder » Thu Apr 20, 2017 10:49 am

Revving it in Park or N produces a smooth - yet stupidly loud - roar that my mechanic swears up and down will have all women in a 5 block radius chasing after me.

Ha-ha, moving right along. Idle in neutral seems smooth, and she sits around 800-900 RPM according to my live data stream. Tranny fluid seems a little darker than normal but doesn't smell burnt, not to me anyhow. This is to be expected imo, since the car has 120k miles and I bought it from my own used car lot for $500. Stands to reason it would need a tranny flush.

The jerking and near-stalls when stopping are lessened somewhat when it's warm, but it still hesitates and lurches when I accelerate from a stop. On the plus side now that those cats are gone I'm not getting catalytic efficiency codes anymore. :D


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