PO302

[HeuiTim]

96 Towncar 150K Miles. Has stumble, miss, trailer-hitching. Get code PO302. Under full throttle problem seems to get better, as more fuel is commanded to offset excess air causing lean miss.

 

Vacuum leak? EGR valve?

 

Previously this car has had DPFE sensor replaced about 20K ago, mass air sensor was dirty and caused a lean code, but this one varies, light low low speed heavy trailer hitching, highway smooths out, with cruise on it almost disappears.

 

I figured you guys have seen it before, the internet 'experts' want to say injectors, burnt valves, plugs, coils etc. I refuse to throw parts at it, I am going to pull the egr valve to see what things look like, poke around the hoses but the fact that under full throttle the car smooths out, power is still good, I don't buy #2 is weak or dead.

 

Any ideas?

[Aaron]

Have you smoke checked it?

 

Try moving the injector, plug, and coil all to different cylinders, noting which one you moved each part to... and then if the miss returns, you can eliminate one, two, or three of the items.

 

What do your fuel trims look like? LT/ST? This is an OBDII vehicle, right, seeing as it's a 96?

 

What's a quickie compression test tell you? (If you roll the motor over in clear flood mode, does it roll over evenly, or have a "fast cranking" spot in it?)

 

I'd at least check the squeeze on #2 and #1 and compare 'em, if your smoke check / fuel trims pan out ok.

 

To eliminate EGR, take a pop can, cut a "gasket" out of it, and put it between the EGR valve and the intake manifold, just to see if EGR IS your problem.

 

I'm not much on gas engine driveability, but that's what I can come up with for right now. Maybe one of the more experienced guys will chime in here somewhere.

[Jim Warman]

Well.... you have a code for one cylinder and one cylinder only. The EGR valve affects all of the cylinders (EGR in what I assume is a 4.6 car isn't ported).

 

I'd start be eliminating (or confirming) that something to do directly with cylinder 2 is it. Inspect the spark plugs and wires, use a spark tester or ignition scope to determine if the wires can carry high voltages, test compression... skipping the basics has hung more techs than you can imagine.

 

Check your fuel trims to see if there are any clues there... mode 6 data to see if any other cylinders are indicating misfires.

 

FWIW.... when you full throttle the car, it drops into open loop. This can alter many symptoms - concentrate on whatit DOES do not what it doesn't.

 

"Throwing" parts at a problem. There are several parts that we consider as "sacrificial"... spark plugs in particular... unless there is a fairly fresh batch of Motorcrafts in there, it wouldn't be a bad idea to "throw" some in. If the concern is fuel related, there is a possibility that a insulator can become etched. When required voltage climbs about some figure, the gap no longer ionizes and the spark follows the etched mark in the porcelain. These etched marks can often be too faint to see with a magnifier.

 

HTH.

[HeuiTim]

blockoff plate idea. I like.

 

Thanks

[Bruce Amacker]

Previous responses are dead on. Just because it's flagging one cylinder doesn't mean it's only one cylinder at fault (necessarily). My first venture would be to look at Mode 6 TID 53/51 misfire and see what the other cylinders' counts are. If only #2 is counting I'd be looking at that particular cylinder. If other cylinders are also flagging I'd be looking at MAF, EGR, vac leaks, etc, or multiple restricted injectors.

 

What was LTFT/STFT on each bank? Does it miss at an idle? Try injecting some fuel to see if it smooths out- propane is my favorite from a BBQ tank, but carb cleaner or flammable brake cleaner will also work. Do this while another tech is power braking it lightly, this is my favorite test for finding restricted injectors.

 

Does it have the fuel injector delivery test in IDS? Do you have an injector "buzz box" for commanding the injectors on? (I bet you do, somewhere)

 

Knowing Ford gassers LOVE to clog injectors, that would be at the top of my hit parade. We had 2 Motorvac machines and made a shitload of money cleaning them. They run great when done.

 

I'm with you on the EGR blockoff but if it's a restricted injector, removing EGR flow might make that cylinder fire again. I wouldn't take it to the bank.

 

Good Luck!

 

[Mbl35]

Usually it's the 4.2l that has the egr misfire problem, when egr is comanded and the egr ports in the intake manifold are clogged with carbon and all egr flows to the unclogged port or ports and thats when the misfire occurs.

On the 4.6l i believe pre obd2 the throttle body pedestal or air horn's egr passage gets cloged up with carbon and causes stalling at idle and flickering of the check engine lt. with an egr code.

 

I'd check mode6, swap ign. cables and spark plug to another cyl, check for coolant leaking into the sparkplug well, a steamcleaned sparkplug, oil fouled sparkplug-it could have a leaky valve stem seal, warn plug etc. If all that and compression is good swap an injector with a good fireing cylinder and see if the misfire moves.

Usually vacuum leaks on these are in the pcv system or evap intake line conection, also i think there is a vacuum line conection in the baclk of the intake that commonly leaks.