Jaguar Repair: 87 XJ6 Starts fine, dies in gear (part 5):, air flow meter, coolant temp

Question
QUESTION: Hi Howard,

This is an update to my Jag woes (http://en.allexperts.com/q/Jaguar-Repair-3784/2008/9/87-XJ6-Starts-fine-6.htm)

I reconditioned the head, lapped the valves, cleaned everything up, installed all new gaskets and valve seals.  Also detailed the engine somewhat and it looks amazing.
Put it all back together and I'll be darned if it isn't doing the exact same thing...runs better than ever, but runs very rough when I put it in gear ...barely moves the car.

I pulled the plugs and found that 1,2,3 & 5 were carbon fouled (black but dry).   I checked compression again with the engine hot. All were in the 135 dry, 145 wet range (140 dry when the engine was cold).  I did not put new rings in since I would have had to pull the engine or remove the front suspension.  To me, it doesn't look like the rings are to blame, anyway.

I looked down the throttle and saw some blackish fuel seeping back down from #1.  Not much, though.

I read that carbon fouling can come from a weak ignition...I had previously swapped coils with the same results. I ran the car in pitch black and didn't see any stray electrical sparks.   Is there a test for the ignition amp?

Also, as a reminder, my fuel pressure was within range with and without vacuum.

Any thoughts?  I appreciate your help (again).
Dave


ANSWER: Hi Dave,
When the plugs are flat black, that is excess fuel. The major influences on fuel mixture are the coolant temp sensor, air flow meter and you already checked the fuel pressure. I have only seen one out of ten series III Jags idle smooth. Every one I made idle smooth, it would not run good. If you don't have CO equipment you can take a volt meter and disconnect the 0 2 sensor and put the volt meter on a 0 to 1 volt DC scale and heat the engine up and read the 0 2 sensor wire (blue wire) the black and the white w/green are the heater wires. The 0 2 reading should be about .5 v. A rich mixture gives you a high reading .7, .8, .9 or 1. volt. Any below .5 is lean.
A rich mixture should have given you good throttle responce and quick power. A lean condition usually gives you a good smooth idle and poor quick throttle responce and low power.
One more item you need to test is manifold vacuum as you slowly open the throttle. If it goes to zero quickly you need to remove one of the plugs on the exhaust manifold and fit a low pressure gauge into the hole and rev the engine. It should never go higher than 1.5 PSI.
If it does, you have either one of the two CATs or a muffler stopped up.
let me know,
Howard

---------- FOLLOW-UP ----------

QUESTION: I did a check on the O2 sensor before pulling the head.  It read .7 volts.

I already replaced the coolant sensor (the one that has the connector like on the thermotime). There's another with two blade connectors that I didn't touch.

The car runs perfect and I can drive it around until the engine warms up, then there is a change in the idle and won't move the car. Revs fine in park, though.  Would a plugged cat or muffler work fine until the engine was hot?

I can still do the manifold test, though.

Thanks,
Dave



ANSWER: Hi Dave,
No, a plugged exhaust is plugged all the time and reacts to the amount of exhaust that is pushed through it so a high load and high RPM is where it is noted no matter what the engine temp is.

A .7 v reading is rich and would account for the black plug reading (providing it is not a gloss black or wet black reading)
The two CATs on a series III car usually can clean up a very dirty exhaust even one with a .7 v reading.

Be careful on ignition timing, as the amount of advance can drastically effect the performance. It is true that a more retarded timing cleans up the emissions but then the car will just about not get out of it's own way. I have talked to several top Jag mechanics on the subject as I ran tests on timing as related to 0 2 volt readings and came up with, little to no change in volt reading from an advanced timing vs a retarded timing but it did show up in exhaust CO reading as Clean on a retarded timing and dirty on a advanced timing and they all felt that the retared timing resulted in heated CATs thus the cleaner readings at the tail pipe.

If you can't locate the cause of the rich mixture you have (.7v 02 and flat black plug reading) you may want to try something we did several years ago and that was to get a Bosch plug connector that fits the coolant temp sensor and make up a socket from a bad injector or sensor so you can add a variable resistor in line (to either side) of the coolant temp sensor. Then put your volt meter on the 02 sensor and dial in .5v and road test the car to see if it is acceptable. A 0 to 1 ohm variable resistor from Radio shack should do.
Another time I had a customer in the mid West somewhere removed the plastic cover from the airflow meter and was able to adjust the big spring so as to increase spring tension on the "L" door thus lean out the mixture. He had already exhausted the limits of the air bypass adjustment and never found any reason for the over rich mixture. He told me he had to do that to several cars and even went to extremes because if you pry the cover off it almost destroys the cover and he could not get a core charge on a new air flow meter so he would drill a small hole in with the pins in the connector and blow compressed air in to blow the cover off thus not have any pry marks then he just adjusted the spring and glued the cover on and glued the hole closed in the plug connector.

I was a mechanics hot line answer man for BeckArnley/WorldParts Corp for over 15 years so I heard a lot of odd fixes on cars from all over the US.
Let me know,
Howard


---------- FOLLOW-UP ----------

QUESTION: Hi Howard,

I finally made progress with my 87 XJ6 (rpm dropped after warming up from 1200 to 400 when put in gear and sputtered/died).  I believe the problem lies in the fuel system (your initial opinion months ago!).  I've replaced the fuel pump with a spare and still had the exact problem.  I also replaced the ECU with a used unit...no help.

With the car idling during warm up, I checked the fuel return under the gas cap and saw a steady stream of gas.  However, after the engine warmed up and started stumbling the steady stream turned into a sputter.  I put the pressure gauge back on the fuel line right before the rail. It reads  31 steady and 40 with vacuum off of regulator.

The last thing I worked on before this problem started was the air conditioning hose going to the fuel cooler. So, the other night I got a piece of straight tubing and bypassed the fuel cooler.  The Jag started idling normally and idled at 800rpm in gear.  I was finally able to pull it out of the garage and around the driveway a couple times with the engine at normal temp.  I still have a stall/flat spot if I give it a lot of gas, though, so it isn't road worthy yet. Any ideas why disconnecting the fuel cooler would have an impact?  

At least I'm making progress!

Thanks in advance,
Dave


Answer
Hi Dave,
31/40 are good readings for fuel pressure. I don't understand the fuel cooler bit as that is the return fuel to the tanks and if stopped up it would have caused extremely high fuel pressure and the fuel pressure regulator could not control the pressure. As far as non cooled fuel it would just cause the fuel in the tanks to raise in temp and pressure which would still be controlled by the pressure valve close to the canister to about 3 PSI (which I think is too high anyway and I lowered mine to 1.5 PSI)

It would have been interesting to see fuel pressure before and after the cooler removal. Also a 0-2 sensor voltage test before and after. (the blue wire, the other two are just heater wires).

I believe you have other things at work messing with the engine. check the timing and check the advance mechanisms. Look for vacuum leaks too.

Also keep in mind that the coking of the intake valves is a common problem with the Series III cars and It can effect the running especially during warm up and at low RPM performance. This usually happens from 60K to 90K miles. Turn the engine over until a intake valve it fully open and remove the spark plug and put a tiny probe light into the combustion chamber and look at the under side of the open intake valve to see if it is coked up. You will have to do several to be sure.

An air flow meter is also a possible cause. Sorry, I don't have a good test for the air flow meter.

Howard