Having a particularly hard time diagnosing a problem, any ideas?

[pureroundcircle]

My 98 Ranger 2.5 has a poor running condition. It stumbles on acceleration (50+% throttle from about 0-25mph), and bucks really bad when trying to accelerate in OD. Idle is kind of lumpy as well. I've looked all over for vacuum leaks, have taken hoses off... I'm pretty sure that isn't part of the problem unless there is some specific location that I might have missed?

I've been trying to track this problem down for months. I replaced a bad ignition coil, but in the process of tracking it down the MAF, and TPS were also replaced.

So.... I'm pretty sure the MAF TPS, and ignition coils are good, plugs and wires have been changed. Scanner shows the MAF and TPS values and they seem good as well.

I have an autoxray scanner, my CE light is NOT on, it never blinks, and no codes are set or pending EVER relating to this problem. It is capable setting a code, if I unplug the MAF, it takes a while but a code will set.

The scanner shows 0 mph for vehicle speed, and 0.00v for both oxygen sensors. My scanner has live data and those values never change, short term fuel trim #1 fluctuates around 0% to +-5%, but short term fuel trim #2 always reads -100%? Anyhow readiness codes read OK for oxygen sensor and heater, wtf? Also ECM runs closed loop as it should.

I've tested the oxygen sensor at the sensor, it works fine. I checked output from pin 60 at the ECM, got 0v. I wired the oxygen sensor to pin 60 directly figuring the problem might be fixed. My scanner still reads 0v for O2 sensors, and problem is still there.

I've also ohmed the fuel injectors, ran fuel system cleaner, and checked fuel pressure, all seem OK.

I'm stumped. I'm thinking bad ECM? but how do I verify? I don't have much cash so I cant really be buying parts I don't need.

Anyone have any thoughts on this mess?

Thanks,
Paul

 

[kimcrwbr1]

Have you checked compression and vacuum pressure it could be mechanical check the health of the engine before you spend alot of time and money on the ignition/emissions. Probably the most neglected part of emissions is the PCV valve change it regularly they cant be cleaned and because it rattles dont mean squat.

 

[kimcrwbr1]

When was your last tuneup including plug wires.

 

[pureroundcircle]

Plugs and wires were changed about 6 months ago. I checked the pcv, I did not check compression yet. I did check vacuum when checking fuel pressure and it seemed alright. I do have a compression tester so I may go ahead and do that; will give me a chance to take a look at plug condition as well, though they should be fine.

 

[BeefStew42791]

have you changed the fuel filter since you've owned the truck?

 

[modelageek]

I've also ohmed the fuel injectors, ran fuel system cleaner, and checked fuel pressure, l

what was the fuel pressure, koeo?, how many cycles to get there?, koer?, how much did the pressure drop in 5 minutes after you turned the truckoff?

 

[kimcrwbr1]

What was the vacuum pressure you should have around 20 and steady at an idle with no more than 1/2 in needle movement. I have done alot of research on PCV systems they are critical in keeping the engine clean (upper end) along with the breather filter keeping dirt out of the engine. Wiggleing it or putting your finger over it do absolutely nothing and there is no real way to clean it. They are cheap and I just change it when I do the plugs as a habit it directly affects vacuum at all pressures.

 

[pureroundcircle]

Figured out the issue. fuel pressure was 62psi, vacuum was 20 in Hg. at idle and at 2500. No fuel pressure drop in 5 min after shutdown, drop to 40psi after about 2 hours.

Anyhow the issue ended up being the almost new plug wires. After running the other tests I decided to run a compression test and check that status of the plugs and found the wires were burned at the connection between the wire and metal end connector, the actual connection to the plugs was fine. These wires are not even 3 months old! I took the wires off the plugs and 4 of them left the metal connector on the spark plug and ash dropped out the end of the connector.

So it was quite obvious the connectors were tight on the plug but apparently I got a new set of wires that was defective and not properly connected.

I also determined that after re-examining the oxygen connector the signal wire was not making a good enough connection for the #1 O2 sensor. So after correcting that it seems to run MUCH better.

Thanks for the suggestions,
Paul

 

[YZ250Rider23]

Out of curiousity.... what brand plug wires were they?

I also had a set of wires rapidly self destruct on my 2.5, and they were Motorcraft wires...