I've got an 86 Bronco II, 2.9L (fuel inj), manual trans.

The other day it started bucking and kicking and in general not running good after getting half-way home (~20 minutes). Here's what I've done so far:

Temp runs normal except after driving 65 mph for ~30 min, then it climbs to the upper part of the gauge.

KOEO code is 11 (OK)
KOER codes 41 and 25

First question; Where is the knock sensor located?

I pulled the plugs and noticed carbon fouling. Put in new plugs. The cap and rotor didn't look so hot either, so they got replaced too. Replaced O2 sensor (didn't measure output voltage with DVM). Inspected/tested/cleaned EGR valve, and found the manifold side very oily/dirty. After initial replacement of O2 sensor I then got only the code 25. Drove the thing for 20 minutes and the same problem (bucking & kicking, no backfiring). Read the KOER codes out again and got 41 and 25 again. Too late and cold to do anymore last night.

Now for the question(s). I plan on testing the fuel pressure (eventually), but because of the delay in symptoms of about 15 minutes or so, could the problem be a plugged (or plugging up) catalytic convertor? I just purchased the vehicle and the previous owner said it needs a new one, but it ran ok and passed emmissions. A month later this starts to happen. Would a plugged CAT make the engine run too rich because the O2 sensor thinks the exhaust is too lean because of lack of flow through the CAT? If it keeps running to rich I can see where THAT would plug the CAT.

Along with testing fuel pressure I plan on testing O2 sensor all the way up to the computer. I searched the forum and found references to using a vacuum gauge to look for an exhaust obstruction. I'm not familiar with that test though. What should I look for?

Thanks much for any ideas or tests I should look at.

 

1st: Usually KOER 25 is a result of not giving the goose test enough of a goose. Make sure when you goose it, to give it at least 3/4 throttle, if not full throttle.

2nd: KOER 41 says that the O2 sensor is indicating lean. The fouling spark plugs suggest that the engine is actually running rich, which indicates that the lean signal is in error. Certainly check the wiring between the O2 sensor and the computer, as you were going to do anyway.

Using a vacuum gauge to check for exhaust obstruction.
1) Hook up vacuum gauge to the vacuum tree at the rear of the intake manifold.
2) Start engine and note reading at idle. Should be 15-20 in Hg, depending on elevation.
3) Run engine at a higher RPM (say 2K-2.5K RPM). Note vacuum gauge reading. It should come back to a similar vacuum to what you had at idle. If the vacuum reading goes down and stays down, that could indicate a restriction in the exhaust.

 

An exhaust leak could give you a false lean code.

 

With 2 jobs and 2 young ones it's hard to get much work done! However, I was able to do the obstruction test with the vacuum gauge. With the car at 6800 ft, I got 17 in. of vacuum at 1000 rpm and the same at 3000 rpm. No obstruction (makes sense). I could understand an exhaust leak causing the too lean code, but haven't found one. Looked through the wiring diagram last night and noticed that on the 86, Ford used a fuse-link for power to the O2 sensor. Anyone know what a heated O2 sensor does when there's no power for the heating? Tonight I'll check the O2 wiring and power. Thanks.

 

Thought I would post the follow-up on what fixed my problem. While checking the wiring from the O2 sensor back to the computer (ECA) I found an orange wire with a frayed end just laying on the transmission bell housing. I Traced it all the way back to the ECA (pin 49) which is shown as a ground on the wiring diagram. I don't know where exactly it was supposed to connect back there so I connected it to a convenient ground point on the intake riser. Now it runs much better! I get a KOER code 42 (rich) and no code 25 with a GOOD goose of the pedal. I'm thinking that the code 42 is due to how fouled everything is from running so poorly for an unknown length of time. If so, how long should I expect to get code 42 from a dirty system before normal operation cleans things up to an acceptable level?

Thanks,
Johny

 

I agree, not sure how long it will take to burn off any carbon build up.