Anybody remember my eight-month saga of fixing up a '93 GMC Jimmy with a leaky transmission? No? Didn't think so. Today was the culmination of that long, frustrating project. I put the fluid in the transmission and drove the truck. It drives great. The tranny shifts smoothly and solidly. I'm very happy with it. I learned a few things, too:

1. A transmission jack adapter greatly decreases the chances of my friend dropping the tranny on my right arm.

2. Letting the powertrain tilt back is easier than jacking the body off the frame but not nearly as much fun.

3. Replace the knock sensor pigtail BEFORE you install the transmission. Fortunately I figured that out before I installed mine.

4. A shop that primarily does tires will not do a good job with a transmission. I let a local shop drop this transmission last summer to fix the leak from the pump seal and they really messed it up. They broke a bolt off the tranny tailpiece mount, they broke the knock sensor plug, they cracked the case, they only put in four of the six bellhousing bolts, they cut the sides out of four of the bolt holes on the exhaust manifold flange with their torch... it goes on and on. I have a music degree and I'm a far better mechanic than those supposed "professionals."

So with that I took the Jimmy out for a test drive and then down to the emissions test station so I could get the new tabs for it. To my horror, it failed the test miserably. Now I have to figure out why before my trip permit expires in two more days.

The Jimmy sat for eight months not running while it was up on jack stands being worked on. Prior to going up on the stands I had removed the vacuum line that goes to the front axle actuator because I was troubleshooting the 4x4 problem. I put a short piece of vacuum hose on the fitting at the back of the engine that the actuator line goes on. I stuck a bolt in the end of that temporary vacuum hose.

It failed high on hydrocarbons and carbon monoxide both at cruise and at idle. I swapped out another MAP sensor on the suggestion of a friend but I haven't taken it for a retest yet. The driving characteristics did not change. I'm not getting any codes. I suspect it's running very rich; I drove it around this evening and according to the gauge it used far more fuel than it should have for the amount of driving I was doing. In the morning I'll see if there are any vacuum leaks anywhere. What else should I check out?

EDIT: I should mention that the IAT sensor was newly installed last summer when I first got the truck. The air filter is similarly new and clean and white. There are no signs of rodent infestation in the air intake tube. There's no carbon buildup on the throttle plate. I ran a bottle of Seafoam through it last summer when I changed the spider and nut kit. The IAC looks clean. I haven't checked the EGR yet.

 

When I put my 94 back together after a long downtime it failed the emissions test. No codes here either. The guy at the test station promised me it was the catalytic converter. So I changed it and it passed with flying colors. If it ends up being your C/C, drive the hell out of it for about 15 to 20 miles before inspection. This too was some advice from the test guy. Hope this helps.

 

Because the Jimmy failed high in each category I suspect that it was running very rich. I was going to back probe the O2 sensor connector to read the value and see if it was putting out an out-of-spec value but the connector was hard to get to. A previous owner had cut the cable on the O2 sensor and welded a nut into the exhaust pipe further back and put an O2 sensor in there. Because of that I just went to O'Reilly and bought another sensor (and another MAP sensor) and replaced them. I also inspected the vacuum hoses coming off the back of the manifold and replaced a few of them.

I took it out for a half-hour drive and it ran well and didn't seem to be running rich. I'm going to take it for another test Monday morning and make sure that it's good and hot. Hopefully it will pass this time.

 

Another test, another failure. I drove the Jimmy for about an hour, including a half hour on the freeway right before the test. It failed high on everything. Hydrocarbons at cruise were 196, the limit is 150. Hydrocarbons at idle were 360, 220 is the limit. CO at cruise was 4.83%, limit is 1%. CO at idle was 3.96%, the limit is 1.2%. I just don't know what to do from this point on.

Any other ideas?

 

I took the Jimmy out to a friend in Issaquah who has a Snap-On scanner. He put it on the truck and checked it out. Everything was according to spec. The idle was 650 RPM, the truck was in closed-loop mode, air/fuel ratio was 14.7.

There are two major possibilities I can think of at this point. The first is that the purge canister is full of gasoline and the engine is sucking it in and burning that excess fuel. The other is that the catalytic converter is not functioning properly and letting all the unburned fuel just pass by it. I'll check the canister and see what I can come up with.

 

Have you checked out your ignition timing? Too far BTDC would cause a rich exhaust, also explain the poor fuel economy.

 

I haven't checked the ignition timing. I'm taking it back to Issaquah on Friday to get more data from the scanner. I seem to recall the short-term fuel trim was a little high (134). I could be wrong about that number which is why I want to verify it. I'll check the timing at the same time. However, it doesn't seem to be running rich since I changed out the oxygen sensor for a new one.

Today I swapped the purge canister with the canister from my wrecked '94 Jimmy and plugged the inlet to the throttle body from the canister and ran it back through the emissions test again. It failed with even higher numbers than last time. Hydrocarbons were up in the 500 range at idle. That suggests to me that the catalytic converter is bad and I should have it replaced. I'll know more once I've scanned it again and conferred with another friend who is a factory-trained GM technician and former GMC Truck dealer.

I appreciate all the suggestions. This is definitely the most difficult thing I've ever had to troubleshoot.