I have developed a problem with my 4.2 at idle. This came about over night it seems. The engine runs smooth while driving, but when I stop at a stop light the engine gets real rough to the point of conking out. It is a 1998 4.2 with 161,000 miles on it.

The CEL came on with the following codes:

P0171 System too Lean (Bank 1)

P0174 System too Lean (Bank 2)

I have cleaned the MAF and checked the IAC. I have tried in vain to find a vacuum leak by spraying starting fluid around the hoses, but I know i'm not reaching everywhere with it.
I heard that you can have a "Smoke Test" done to find a vacuum leak. Has anyone heard of this before and where can I get it done? Dealer?

Any help finding a solution to this problem would be appreciated.

 

Good luck finding someone to smoke test ! I've seen & heard of it, but never heard of a place that actually does it.

My best guess for the miss (assuming it gets tuned up, new plugs, etc.) would be in the EGR valve hanging up. I've seen it often on GM product, where carbon buildup falls free and sits between the pintle of the valve and its seat, creating an effective vacuum leak, feeding the engine with spent exhaust gasses. Can't say if this would create a lean or rich condition, though. If you drive at highway speed, the engine can consume exhaust gas without a driveability issue, but at low speed it screws up the carburetion. Then the ECU tries to compensate by varying fuel ratio, etc.

Hope this helps.

Denis.

 

You can look here for additional information.

http://www.v8sho.com/SHO/TSB0197HO2SServiceTips.htm

 

I had this code appeared with my 98 V8, Sent it to the dealer and they told me they need to replace the intake manifold gasket and quoted my $900 for the parts and labor.
I bought the parts myself and installed the gaskets, turned out the old gasket was in good condition and it was not the problem. The problem was a small hose has deteriorated at the back of the engine near the firewall. It cost me just $37 to replace the whole hose assembly.

 

The infamous PCV hose is the most common reason for the P0171 and P0174 combo...

FWIW, my local Ford dealer has a smoke machine. Had to avail myself of their service when my 98 Contour had an evap system leak. Call around, you might be surprised.

Steve

 

ok i kno dumb question, u sed u sprayed the gasket for leaks at the IAC. but did u take it off and clean it? the plunger could very well be stuck in close to wide open (highest throttle for idle) if u havent.

 

Does it only exhibit the problem when the brake petal is pressed? If so, then it's probably a bad brake booster. To test it: Get a pair of locking pliers and pinch off the hose going to the brake booster and test drive it. The brakes will be more difficult to manage than usual, so be careful. If this seems to have fixed the engine's missing problem, then it's a bad brake booster!

 

Thanks to all who responded.
I will check the PCV hose, I didn't get a good look at it before. It runs rough in park and when I have the brake on at a light. It just runs rougher when it is in gear. Good point racerf150-2, i will clean it. Thanks for the web link temp1 good info!

 

I'm amazed {well maybe not} the dealer would misdiagnose the problem. What are the techs learning in mechanic school that they would come back with a $900.00 repair when all that was needed was a $37.00 part? Geeessh.

 

Visually inspect your vacuum hoses for cracks and splits. Also ensure your air filter inlet piping is intact and aligned as a gap will allow more air coming in and throw off your MAF sensor reading. Good luck

 

I have also been advised to change the O2 sensors. Since both banks created errors I can't see them both going out at the same time. One I could understand, but both at the same time???

 

I changed a melted 90 that went from something back there to the rear of the intake. Reset the computer and started the engine. It ran worse than before, but the CEL stayed out. I swear I hear a vacuum leak, but when I spray carb cleaner around nothing happens. I also took the IAC off and cleaned it. Check it with a meter and it is ok. Thanks for hte help!

 

Did you try what I suggested?

 

FIXED!
The 90 under the IAC was melted as well. I made a temporary fix with tape until I can get to the dealer for a new one and the engine is running like new again.
Thanks to all for your suggestions.

 

Congratulations for your fix! So you also got those hose assembly problems