P0171/P0174 Help!

Well, I sprayed some B12 around the IM while the engine was idling and didn't note any detectable RPM change. The intake is stock and there is a "normal" filter in place. I did try a K&N filter way back when before deciding it was of no value. That was a long time ago and the MAF has been cleaned with CRC MAF cleaner since then. Another freeze frame had RPM at 2521 and MAF at 7.152 lb/min.

How are your brakes working?

Why would his brakes cause a 2 lean trouble codes?

Vacuum leak through the booster?

Seemingly just fine. They are due for service when I find a free weekend (really when I have the ambition to press out studs), but no indication of sticking. Last time I put the jack under the truck and tried turning the wheels by hand there was no apparent drag, which is what I presume you're getting at. I'll check again when I get a chance a bit later today.

The devil is in the details.

True, Scratch the booster leak.

Not really much for vacuum lines on this truck, since it has hydroboost and they long since stopped using vacuum actuators for stuff like HVAC.

I did find another issue when farting around with it yesterday. While the engine was idling I pulled the fuel pump relay and to my surprise watched the fuel pressure slowly drop until the engine died. Seems that the fuel pump oil pressure switch isn't doing it's thing. The oil pressure gauge reads normally and it's a combined switch / sensor unit, so presumably the oil pressure is fine. There doesn't seem to be an easy way to connect a direct pressure gauge to validate. I need to connect a meter to the wiring near the fuel pump and see if it's suffering a voltage drop under load due to perhaps burned relay contacts.

I also disco'd the fuel filter and checked it yesterday. The fuel filter flowed freely, so it was reinstalled when I was done (did have another filter on hand). While it was out I connected a hose to the supply side line and ran it into a gas can. I put my manual switch in place of the fuel pump relay and powered it up and it seemed to pump more than enough volume.

OK you said you had access to a Snap-On scanner. Do you have access to a scope? If yes connect up to the fuel pumps power and make sure your not losing any voltage or amperage through the line. Either one could cause the pump to slow enough that it could be starving the engine.

I'll trade you problems. I have a 96 Jeep in the shop. with a NO run problem. Connected the scanner and it can't connect to the PCM. Checked for proper power and grounds and they are good. Stuck a new PCM in and still no joy. Started looking deeper and so far the only common thing is that the PCM isn't working and there seems to be no reason for it. Jumped around any switches or relays and still nothing. So now I'm going through the harness looking for a problem.. It's only 106 wires though....

I'm coming in a little late here, so someone might have already mentioned this. Pull the pcv hose off and the breather hose off the valve cover and see if the engine is pulling a vacuum in the crankcase. Sometimes the intake gaskets fail internally and suck from inside the engine.

No real time scanner available. I did spray B12 around the intake and monitored with the calibrated ear for any RPM change and detected none.

No mysterious coolant leaks. The only leak is the block heater port that dips a few drips per day and only during the winter when things are cold and contracted.

The Snap-On scanner is one of the fancy graphing, scopeing, coffee making ones :) I don't need it for those checks though, I can do those with my Fluke 87.

Connectors, it's always the connectors, especially in a harsh auto environment... well, at least it is when it isn't a mouse eaten wire :(

Good vacuum at the end of the PCV valve when it's pulled out, no vacuum (or pressure) detectable from the ports in the valve covers.

I get 12.7V at the fuel pump connector by the tank vs. 13.8V at the battery terminals. I cleaned and tightened down the fuel pump ground terminal while I was at it, but it was plenty solid to begin with. I also popped the cover on the fuel pump relay and the contact look perfect.

Not possible on this one. It has a mechanical gauge setup. I may grab a new crank sensor and see what it looks like on the meter. The book doesn't show electrical values though.

I was just about to say, "its a 96 Jeep, replace the crank sensor." :-p

Had a new one last week. Driving home from vacation in the 99 Jeep, after a gas stop got a CEL and intermittent cutting-out. Not pleasant

Turned out to be the THROTTLE position sensor. So much for the goober who was telling me how most cars don't even need the TPS to run a few weeks ago :-p. Well, it did run 60 miles to an open parts store... but it wasn't much fun.

And it was a time I was glad to have a cheap OBD-II reader (sucker tool ;-). I was expecting an intermittent crank sensor loss by the way it was acting, but that wasn't it.