1,2 and 4 running rich

[HappyDog]

Have you done a compression or leak-down test?

No, I didn't think that would be necessary because the vacuum gauge fluctuates only when there is a miss, but is otherwise quite steady.

 

[Ecomike]

I remember that thread about the valve springs causing the missfire!!! Very interesting.

 

[Sidewinder CC]

I noticed on one of the Youtube videos about O2 sensors that one in good condition will respond quickly enough that at idle speed one can see the effect of each cylinder as it's exhaust valve opens. So I thought this would be a good way of testing whether the richness was due to the injectors or something else. I used my pocket oscilloscope to look at the output of the B1S1 O2 sensor and was amazed that even with this old sensor I could see the effect of the lean cylinder, even though I could not see the rich cylinders individually.

I also used the oscilloscope to look at the injector drive, and each one looked good in the sense that the pulse came down very close to ground for each injector. So I'm pretty confident the injector wiring and PCM grounds are good.

I plan to combine joe_peters's and Sidewinder CC's comments about swapping injectors and determining which cylinders are lean by swapping injectors 3 (lean) and 4(rich), which will put all the rich injectors in bank 1 and the lean injectors in bank 2. I will again view the oscilloscope traces and see if swapping the injectors changed the patterns -- if so, the problem is the injectors.

Excellent troubleshooting approach. It sure is nice to have the proper tools for the job (oscilloscope).

If the patterns are unchanged after swapping the injectors then something else is amiss, either with the the spark or the cylinder itself. For instance, Dcoy1283 (http://www.naxja.org/forum/showpost.php?p=245752303&postcount=17) found his misfires were due to the valves not closing well and the misfires went away when he replaced the valve springs.

The injector swap test will have to wait until I get replacement o-rings for the injectors. I'll post the results when I have them.

I used Felpro injector o-ring kits from Advance Auto, they worked good for me.

 

[HappyDog]

It's time for some updates.

I've done a bunch of things and I still haven't found the real problem.

Several people suggested I do a compression check. Here are the results:

2001 Cherokee
1 170
2 185
3 170
4 180
5 185
6 185

I don't see anything there that points to a problem.

I replaced all four O2 sensors, the two main reasons being
1. The claim is that they only last 100-150kmiles, and
2. I needed to get the car smogged and didn't want any problems.

I wasn't so sure about reason #1 because I recently smogged (and passed with flying colors) a 1993 Camry with 253 kmiles, and it has never had its O2 sensors replaced.

Long story short:
1. the old O2 sensors looked pretty good
2. The new sensors made no difference to the gas mileage nor to the miss at idle
3. the car passed the smog test easily

So let's back up and rethink the problem. The issue is declining gas mileage. That can be due to
1. Engine inefficiency
2. Accessory drag -- all the pully-driven stuff
3. Driveline drag -- torque converter, transmission, bearings, differential, wheel bearings, dragging brakes, etc.
4. Fuel loss due to leaks, evaporation, etc.

The engine appears to be operating well even though it has the little miss. I recently purchased a 1997 XJ and it has the same little miss, but gets better mileage.

Accessory drag -- I think there is something here because the serpentine belt tends to "sing" -- not squeal as in slippage, but sing as in a taut wire being continuously plucked. I have tried taking off the belt and turning things by hand. Nothing seems to grate or give evidence of bad bearings, but I can't turn them by hand at the same speed and with the same load as the serp belt. I have seen Joe Peters comments about using a hose to listen for problems, but it's really pretty tight down there with the mechanical fan, the belt looping back and forth among all the pulleys, and there's a lot of momentum there to grab that hose and throw it where it will do the most damage. So my attempts with the hose haven't been effective.

Oh yes, I did change out the belt -- actually went back to the old stretched one, but it sang also.

I have read about others finding the various accessories creating significant drag as their bearings wore out -- JP magazine has a nice 3-part article on trying to keep decent mileage. But I don't want to replace parts without knowing which one(s) is/are the culprit(s). AC clutches, water pumps, power steering pumps, alternators, and so on get pretty expensive.

I'm trying to figure out how to measure the "accessory load" the motor is experiencing so I can tell if it is normal or becoming excessive. That's one reason I bought the '97 just to have something to compare to.

As for the drivetrain I don't see anything there that concerns me. The brakes are not dragging and the rest seems to operate fine. I was surprised at how warm the differentials get at highway speeds -- I'll have to instrument both Cherokees and see what, if any, difference there is.

I haven't seen any evidence of fuel leaks, and the smog test is supposed to make sure the vapor recovery system is working well. And I have a locking gas cap to discourage would-be siphoners.

When I had the O2 sensors replaced the mechanic said I was stessing too much over these things that are working pretty well. But he's not the one paying for the gasoline!

It's not only the price of gas, but the sheer inconvenience of having to break off exploring back areas in order to go to a gas station again.

Thanks for all your suggestions...

--HappyDog

 

[IH392]

Another item to ponder is that maybe one of the precats is clogged. I don't how you could check that though. I know you can use IR gun to check their temps and see if they are working. But I would think the bank that is running rich would have the precat that is running hotter.

 

[Talyn]

You could do a back pressure test on each cat using a vacuum/ pressure gauge to see if there is any significant restriction.

What about a leaking intake manifold gasket on cylinders 3, 5 and 6 making them run leaner?

 

[HappyDog]

Another item to ponder is that maybe one of the precats is clogged. I don't how you could check that though. I know you can use IR gun to check their temps and see if they are working. But I would think the bank that is running rich would have the precat that is running hotter.

The two banks are
1,2,3
4,5,6

The cylinders that are running rich are 1, 2, and 4 -- they are not confined to one bank/precat.

The best explanation I have heard so far was from the mechanic -- the waste spark system pairs
1 & 6
2 & 5
3 & 4

He claimed that one side of the waste spark system is more energetic than the other, so if the "energetic" sides of the 3 coil packs are 3, 5, & 6, the less energetic sides would be cylinders 1, 2, &4.

I don't know about the "energetic" argument.

 

[HappyDog]

I plan to combine joe_peters's and Sidewinder CC's comments about swapping injectors and determining which cylinders are lean by swapping injectors 3 (lean) and 4(rich), which will put all the rich injectors in bank 1 and the lean injectors in bank 2. I will again view the oscilloscope traces and see if swapping the injectors changed the patterns -- if so, the problem is the injectors.

I did get the Felpro o-rings as suggested by Sidewinder CC and did the injector swap. I need to probe the new O2 sensors with the oscilloscope to see if there's any change.

I just got so busy with smogging cars and other stuff that I dropped the ball on this one.

 

[Talyn]

He claimed that one side of the waste spark system is more energetic than the other,

While it is true that one plug gets more energy than the other, its not isolated to either side of the coil. The cylinder with compression uses the majority of the coil's energy and the cylinder on the exhaust stroke uses very little. The cylinder with compression has a higher resistance and requires more energy to fire the plug.

 

[HappyDog]

While it is true that one plug gets more energy than the other, its not isolated to either side of the coil. The cylinder with compression uses the majority of the coil's energy and the cylinder on the exhaust stroke uses very little. The cylinder with compression has a higher resistance and requires more energy to fire the plug.

Yes, after I wrote that I looked up how the waste spark system operates and discovered exactly what you say. However, the mechanic was trying to make a different claim, namely that one of the paired plugs generates a hotter spark when it is on the compression cycle than the other plug. The claim is that because the center electrode of one plug is positive and the other plug is negative, the spark is better-formed on one than the other, and thus ignites the mixture more effectively.

I did learn the plug whose center electrode is negative erodes the ground electrode more quickly than normal -- that's why one wants to use double-platinum plugs in an engine that uses a waste-spark system. This is all explained in DYK_Waste_Spark_Ignitions.pdf

 

[Talyn]

Yes, that is true that one plug fires positive and the other fires negative. I was going to mention that, but I also could not find an internal schematic for the coil rail so I have no way of knowing which one fires which way.