Running rich at high RPM

[tomj76]

I have a '96 with the 3.8L V6 Engine. I have 315k on the engine.

When running at idle, the scan tool shows a "normal" trim, slightly lean (+5%). However, at 2000 rpm it changes to a negative trim (-15%) to compensate for a very rich condition. I've been getting bank 2 rich codes. It also blows black soot from the tail pipe at 2000 rpm.

I know that I'm burning oil, at about 1 qt every 200 miles. There is oil found in the pleats of the plenum between the throttle body and the PCV pipe. This could account for the rich reading, if the oil is "counting" as fuel by using a portion of the O2 entering the combustion chamber. I've also found severe oil deposits on the plugs in cylinders 4, 5 & 6 to the point of causing misfire.

I use it mostly for short trips, which probably is a factor in the amount of oil used. I've switch to 10W40 to improve the oil consumption somewhat.

Although I'm planning a complete overhaul of the engine (why wouldn't I at 315k miles?), I'd like to identify the "big pole in the tent". I tested compression and saw good wet/dry compressions (in spec) of ~190 psi dry, ~210 psi wet. Cylinder 2 was a bit low the first time I tested is (150 psi dry), but I reteseted it the next day and it was better, even better than the wet test than the previous day.

I also measured leak down, and found that all cylinders were better than 20%, with most of the leakage detected on the oil dipstick tube.

While I suspect that the problem is likely valve guides & seals, I'm wondering if anyone can explain why it runs rich at high RPM. Is this evidence of a bad oil ring, or something of that nature?

[wiswind]

I don't have much to offer other than getting oil in the big flex hose between the throttle body and the air filter unit is normal.
One thing that can help this is to use a genuine FORD/MOTORCRAFT PCV valve so that you know you get the correct flow under the various conditions.

In the picture that my signature takes you to......you can see the "catch can" that I installed in the run between the PCV valve and the throttle body.
I gave a long run of hose to try to give the vapors a chance to cool down some.
The catch can helped, but I still got some oil in the summer.
I did not have an issue with oil consumption.

Is the Intake Manifold Runner system operating properly?

Along with my thought on the IMRC.......have you removed the upper intake manifold and cleaned the EGR ports in the lower intake manifold lately?
They can be partially clogged and causing issues BEFORE you get any Check Engine Light.
These ports tend to clog from the driver's side FIRST....working toward the passenger side.
What this does is gives more of the oxygen starved EGR air to the cylinders on the passenger side vs the driver side when the EGR valve opens.

If you have not had the upper intake manifold off in a while.....you may be surprised and how much black crud builds up on those IMR plates in the lower intake manifold.
If you clean them.....you will need to AVOID letting that junk and/or cleaning fluid from running on down below them to the intake valves.

Another thing that I needed to do with my '96 from time to time was to run Berryman's B-12 Chemtool (in the metal can) through the fuel system a couple times a year.
I tried other cleaners......even in maintenance dose with EVERY fill up, but STILL needed to run the solvent based Berryman's once and a while.
The issue was that I would develop some buildup inside the cup that is over the spray end of the fuel injectors.
Of the MANY products that I tried.....Berryman's was the only one that worked for this particular issue on my '96.
The way a fuel system cleaner works BEST is if it is in the fuel system for a while.
I used to put it in before going on a trip.....so that I had a constant flow.....but read that the best result is when it is in there.....soaking over time.....say a week or so.

[tomj76]

Thanks for the suggestions. I did find that IMRC on Bank 2 had come loose, and replaced the clip to secure the linkage. However, after that repair I still see the rich reading at high RPM.

I did a vacuum gauge test, but didn't find anything out of the ordinary there. The vacuum at these higher RPMs is about 21 inches vs. 19 inches a idle. I also ran the engine with the PCV lines disconnnect from the intake, mainly to see how much blow-by gas is returning to the intake. It wasn't much, but a steady stream of smokey gas was coming from each hose, especially at the high rpm, and mainly from the Bank 2 line (which is understandable, as it has no PCV). I checked the scan readings while I had the PCV lines disconnected and saw that is was indicating a slightly less rich condition (by ~2%).

It has been a while since I last cleaned the EGR ports, probably 18 months, and it is worth recleaning to see what impact that has on this reading. I'm in favor of checking this thoroughly as possible before tearing down the engine for rebuilding, just to make sure that I know where to focus my attention to correrct the problem areas.

[scubacat]

The other IMRC clip is probably broken as well. Mine looked fine, but when I just barely touched it it practically disintegrated. I'd replace both (especially considering it's 5 bucks for a 5-pack anyway..) The middle ones are almost never an issue; it's the outside ones that the lift the arms.

[tomj76]

The ports were clogged, but cleaning them didn't correct the rich condition at high RPM.

The Bank 1 IMRC bushing is OK, but I agree to replace it with one of the spares at some point.

[tomj76]

I checked the MAF... it seems that contamination on the MAF sensor wire can make it report too much air at low flow rates, then too little air at high flow rates. Initially cleaning did not make any difference, so I plan to repeat to see if the results will improve. Both banks report rich conditions at throttle, so it seems that the problem is due to a common component.

[tomj76]

So, as I have been detoured from doing any direct work on this, and I've been trying to understand how it can be lean at idle but rich at high rpm, I came across this google book:

http://books.google.com/books?id=K0K...ide%3F&f=false

pg242 shows how a worn valve guide can pull valve cover gases in through the EXHAUST valve. In relating this to my problem, any air leaking down through the exhaust valve will skew the oxygen sensor to believing that there is not enought fuel in the combustion chamber, and the PCM will react to add more.

When I checked with a local machine shop for quotes on rebuilding the heads, I asked the machinist what he thought about the fuel trims that I'm reading and he suggested that the catalytic converters might be "clogged" with oil soot. I suppose I would get the same effect if an exhaust valve is caked heavily with carbon build-up.

[tomj76]

I sealed a small plastic bag over the oil fill port (with the cap removed) so see if there was positive or negative pressure in the crankcase. With the PCV system hooked up it was under a slight vacuum at all RPMs, except in the moment right after opening the throttle suddenly, then the bag filled with air. With the PCV system disconnected (and appropriate plugs on the intake), and a bag over the pipe that attaches to the upper intake manifold, the bag over the oil port fills with air, about the same regardless of throttle position. The bag on the PCV pipe smokes. The system also continues to run rich under those conditions.

Today I removed the lower intake. The intake ports are all coated with a sooty/oily cake material, about 0.05" thick. This material is caked from the upper intake all the way down to the valves where the aluminium can be seen around the valve stems.

I don't know what it means, except that this can't be directly from the valves/valve guides. I suppose the soot could be from the rich mixture returning through the EGR, and the oil could be from the PCV system.

[tomj76]

I've removed the heads and the oil pan. All that's inside the engine bay is the block, front cover and the water pump. I could probably pull it out through the hood at this point if I need to.

The "right" head (4-5-6) valves were heavily caked with carbon and combustion "stuff" (the brown/tan deposits that build up on the exhaust valves). The valves on the oil burners were the worst by far (4 & 5). The valves move around in guides a fair amount, some as much as 0.010". I pulled the #5 piston, and checked the ring gap on the top compression ring, and it's 0.045" compared to 0.015" max from the service manual specs. The cylinder diameter measures very close to the original dimension (3.81"). The left head is in better shape, with less build-up in general. I have not yet checked the valve guide clearance.

The lifters were heavily caked with oil deposits. There was a pile of stuff under the pushrods for cylinder 4.

The crankshaft main bearing clearances were measured a year or so ago, and they were only 0.002 with much wear on the bearings. I'll recheck that. I don't think I "need" to regrind the crankshaft. The journals aren't mirror smooth, but the striations don't seem very deep to me... I have trouble with that one.

I also took the EGR apart, as much as I could. I want to see if the valve is working right, so I'll be look at that as I work through this rebuild.

[tomj76]

I got the EGR disassembled, the valve works, but it doesn't seal tight. I'll purchase a new one.

I also have my rebuilt heads back... the found small cracks between the exhaust and intake valves on the 4-5-6 head. They don't seem to go past the valve seats, so I think that part is OK. They did not replace the valve guides, said they were OK. I removed the spring from one of the worst ones to check for play, and it was much better than before. The leak checked them and resurfaced the block face. I'll need to do something on the intake faces to deal with corrosion pits.

I'm trying to get the front cover off, but there are two studs that won't come out of the block, nor can I get the front cover to move. The ones I was able to remove were rusted, so I suspect these two (which happen to be right next to the water outlet passages) are rusted tight. I'd very much appreciate any suggestions as to how to get these out.

[tomj76]

I was able to remove one of the studs by pushing the cover back on, then turning the stud with visegrips. I still can't get the other one to budge... I broke the threaded part off trying to remove it with two nuts tightened against each other. I tried to heat it with a propane torch, but I couldn't get it very hot with my torch.

[tomj76]

OK. I was able to remove the front cover... I applied heat to the cover while tapping on the cover at a flange. The corrosion had filled the space between the stud and the cover hole.

Now to find replacements for the broken studs. It would be nice to replace the whole set since they all have some corrosion on them.

I think it may be possible to remove the front cover while it is still in the vehicle if you remove the studs from the front of the block before extracting them from the cover. I didn't have a chance to try that due to the froze studs.

[tomj76]

FYI, once I had the heads removed, and all that remained in the vehicle was the front cover and the block, I decided to remove the block so that I could do the rest of the work with the block on my engine stand.

I bought a "re-main" kit, which includes all the parts needed to replace the bearings, rings and gaskets. Since I didn't have the crankshaft machined, I purchased a standard set of main bearings and connecting rod bearings. I think there is an option for 0.025 mm oversized, which may have been a good idea. I measured the new bearings with plastigage and found that the are within spec.

I found that I have a problem with my inexpensive caliper set (I have a Storm 3c301 digital caliper). It seems that it measures about 0.003" - 0.004" smaller than the actual value, at least for the cylinder bore measurements that I took. I checked the readings with a better set made by Mitutoyo and found this discrepancy. There's also a different reading for the inside caliper compared with the outside caliper. Consequently my standard sized rings came up with more gap space than allowed. The bores measure straight and round.

Both seals have worn grooves into the crankshaft and harmonic balancer, so I will install repair sleeves to correct any oil leak due to the poor seals.

[olopezm]

Those fuel trims are very weird. Have you checked the fuel pressure regulator and fuel pressure? I recon that one would make the engine run rich overall but I guess is worth a shot.

A bad/dirty MAF would cause negative trims at idle, normal at cruise and high at heavy load.

A cat converter would become clogged because of the rich mixture and oil deposits. That would show up as a lack of performance and the engine choking because of it's inability to produce vacuum. You would see that with a vacuum gauge but since your readings seem normal and don't drop to 0, the cat is still flowing.

Oscar.

[tomj76]

Oscar:

Those are good points.

I checked the fuel pressure at idle and at 3000 rpm, and was in spec.

I decided to clean the MAF, just in case it would help, but nothing changed.

I checked vacuum as well, again at normal idle, then at 3000 rpm, but it recovered to the normal vacuum level.

I also checked the O2 sensors for proper operation, which in my case means that I put them in a vise, applied heat with a propane torch while monitoring the output voltage. I saw the expected readings (0 volts with no flame, ~1 volt with the flame).

A discovery I made while disassembling the engine was a crack in the upper intake manifold at one of the barbed fittings along the back of the manifold. I suppose that could have been the cause of the lean reading at idle.