QuestionHoward:
I have included our correspondence from 8/12/2008 below for background. Thanks again for your help. I got the cam covers off today. The head looks very clean and the tappet guides appear to be pressed in to a uniform depth and are undamaged. I measured valve clearances, hoping to get good readings. Unfortunately, the readings ranged from a high of 0.022” for #3I to 0.010” for #6I. Plus three of the plugs are sooty black. Given these readings and the poor compression test results, with only 13K on the car, I assume it makes no sense to mess with adjusting the clearances and I’ll have to remove the head to have it rebuilt. Please let me know if you think differently.
If the head needs to come off, I would appreciate your advice including tricks of the trade. It will be a solo job, but I do have a hoist and will plan to remove the hood to make it a little easier. If the studs are frozen, should I use your 2-nut approach to try and get them out, at the risk of breaking them? Or should I try to get the thing off with the studs in place? I assume I can get the AC stuff out of the way without breaking into the coolant system. Please let me know if that’s not possible.
There is a machinist/mechanic nearby who has a good reputation with Jaguar engines; unfortunately I would imagine the machine work alone is going to cost upwards of $1,000. That brings me to one of your prior recommendations which was for the guy to consider replacing his shot XJS V-12 with a 350. For a long time, I have been thinking about doing the same thing with my 4.2. I can get a ’94 donor car with a 125K mile LT-1 for about $2,500. And assuming the conversion kit will cost $1,500, I could be looking at a more reliable engine/transmission combo at a net of $4,000 – $1,000 , or $3,000, less whatever I can get for left over parts. Would appreciate your opinion on this.
Thanks Again!!
(Background Info from 2008)
Your Question was:
QUESTION: I am the original owner of a 1987 XJ6 with 13,000 miles. It ran fine until recently. Now it starts fine in park, but unless I pump the gas pedal, it idles rough, slowly loses RPM's, sputters, and finally dies. If I start it again and hit the gas pedal raising the RPM's to about 2K, it runs smoothly without missing. If I start it and shift into drive, it immediately dies. If I start it, gun it in park and shift it into drive, it will take me down the road, but at much reduced performance, though without missing. I checked for obvious vacuum connection issues, jiggled the ignition wires, but have done no real troubleshooting. The fuel filter is original. I would appreciate your ideas on where to start. Also, if you recommend a fuel system pressure test, would like your recommendations on where to get a suitable gauge. Thanks!!
ANSWER: Hi Bill, All of the symptoms are useless information other than to tell you you need to run tests. Yes, fuel pressure is the first I would run. You need a gauge that can read fuel injection pressures. On this car it is about 32 PSI normal and about 42 PSI with the vacuum pulled off of the regulator. While the regular fuel injection testing gauge is expensive you can just get a industrial gauge that can work with fuel and a "T" and some clamps much cheaper. You need a 12v test light and a volt meter and also a "Noid" light available at most auto parts stores (not expensive). Also get a compression gauge.
First run a compression test even though the mileage is low you don't want to leave stones not turned over when looking for a fault. Then run the fuel pressure test. Test the Injection coolant temp sensor with the ohm meter. I found that most operate at about 1 K ohms at 63 deg F. and about 100 ohms at 190 deg F. Check every injector wire with the Noid light to see that all are firing. Clip a timing light on each plug wire to see that each flashes the light. Set the timing as per the sticker under the hood. If everything shows normal (but ONLY after all the tests are run) and it still seems wrong try adjusting the idle mixture and Idle speed after it is warmed up. Let me know, Howard
QUESTION: Howard; Thanks so much for the quick response. I appreciate that. Well, I believe you have put me on the right track with your suggestion for a compression test before anything else. The results do not look good. Both dry and wet tests read the same: #1-130, #2-150, #3-50, #4-50, #5-150, #6-140. I am mechanically inclined and have worked fairly major maintenance on American V-8's. Is there anything I can do, or must I entrust the next steps to the experts? Thanks again!! Regards, Bill
ANSWER: Hi Bill, If you had the throttle open on the test of #3 & 4 and the wet test was about the same, it is an indication you have either a head gasket blown between 3 & 4; or both have valve problems. Check the valve clearance on both cylinders and if they are ok the head needs to come off for either gasket or valves. If you decide to do the job yourself let me know and I will give you a few tips that make it different than a domestic engine to do. Howard
Howard Thanks again for the immediate response. Yes, the throttle was blocked open when we did the compression test. When cranking for the test on #3 and #4 the engine spun relatively easily, and we really didn't get any reading on the gauge, in contrast with the tests on the other cylinders. We will remove the covers and do the clearance checks this morning. I will plan to do all the work myself, so would truly appreciate any tips you may have on next steps, as we go after the gasket or, worse yet the valves. I ordered all sections of the Series III Service Manual when I purchased the car, but have nothing else to go on. So any advice you can offer will be invaluable. Thanks again! Regards, Bill
AnswerHi Bill,
Thanks for the background conversations as it clears up a lot.
Jags are noted for the studs sticking in the head, making it difficult to impossible to remove the head without first removing the studs. This too can be a problem as the studs extend to the bottom of the block through coolant and the Jag suffers from electrolysis which erodes the metal of the studs close to the bottom and when you try to remove them, they break off. The trouble is that if you don't remove them and they are eroded they can break anyway during use which causes more problems as that can happen out on the road. About one engine out of every three that I have pulled down have shown signs of erosion of the studs. Most come out without breaking off though. The few that did break off I was able to get the broken piece out by making tools to do so. If you get that, let me know and I will cover in detail how I made the tools. The front two and rear two are short and only extend into the top of the block.
The low compression not raising when the "Wet" test was run is an indication of either a valve problem or a head gasket. You can sometimes test for it being a head gasket by putting that piston at TDC of the compression stroke and blow compressed air into that cylinder while the spark plug is out of each cylinder next to that cylinder and note if air blows out either of those cylinders on either side as the head gasket rarely blows out badly on the sides but usually blows into the next cylinder.
I take it you have a service manual as you should not try to pull the head without one. My normal procedure is as follows.
Drain the block via the drain plug in the side. Set #1 & #6 at TDC. Unlock the easy to access cam bolt lock tab and Stuff shop rags down into the chain guide area below the cam sprockets and remove just that one cam bolt on each cam and then remove the shop rags and rotate the crank around one turn to TDC again and stuff the shop rags back in and unlock and remove the remaining cam bolt. Then pry the cam sprockets off of the cams and slide them toward each other and if you don't have the clips to secure the sprockets in place just run a piece of safety wire under the bridge in the front of the head and secure each sprocket toward each other. The rags are so you don't drop a bolt down into the cam chain area as you will regret that. Then remove the oil line banjo bolts at the rear and catch the copper gaskets. I usually don't try to remove the intake manifold but do remove both exhaust manifolds. Don't forget the nuts at the front on top and the ones at the front on the bottom.
I take as much junk off of the intake manifold as I can get off.
The head gasket itself can stick very hard so I usually use an engine hoist to lift up on one end of the head then switch to the other end to break the gasket loose. If it don't budge, I then double nut the studs and remove them crossing my fingers that they won't break off. They have a 45 degree seat at the bottom so they will usually be tight for the first move.
Over the years rebuilding Jag engines I have had every combination of problems that you can think of. I have had the studs so tight that double nut didn't work and I was forced to destroy the stud with a stud remover tool. I have had some not break loose and the head only came up about an inch so I had to use a saw to cut the stud off. luckily only a few broke off down in the bottom of the block.
As for a 350 Chevy conversion, I have never done one but have assisted a friend do several but usually on the V-12 and very few 4.2 because the 4.2 is inexpensive to rebuild. (if you are doing it yourself) We only did one late model Vet engine and would never do another as it was a nightmare as compared to a carburetor 350. The Dayna rear end in a Jag is geared about right for a 350 Chevy. As I remember the right exhaust manifold was ok but the left one was a monkey puzzle.
Howard
