Ok, I give up. I've searched for over an hour and can't find the spec (in PSI) here. Anybody have it handy?

Thanks,

Ken

 

Ken:
There are rules of thumb, but the compression ratio times the atmospheric pressure is the theoretical "spec". So, an XJ8 has a compression ratio of about 11 to 1, I THINK, so you might expect a compression test value of about 160 psi.
Oops- I left off the specific heat index- actual maximum stsic pressure is the beginning pressure (1 bar) times the compression ratio raised to the power of the specific heat index (1.4) for air. So , for 11 to 1, the maximum ststic pressure would be narly 280 psi!! In fact, that is lowered by the valve overlap portion of the compression cycle. And the actual compression ratio of the AJ26 NA is 10.5 to 1.
For standard design engines, after factoring the compression and the valve overlap, the measured compression pressure is 15 to 20 times the compression ratio. So, the AJ-26 should be between abot 150 to 200 psi. Most important is balance between cylinders.

 

160 psi is good. What you are looking for is similar readings for all your cylinders. 120 psi can run you down the road. As the engine wears it drops.

The 11to1 compression ratio is misleading. It is looking at the total volume with the piston at bottom dead center and seeing what you have at top dead center. However, if your intake camshaft is set to close the intake valve somewhere past bottom dead center (normal) then you may have in effect a compression ratio that is the same as 6.8 to 1. This is one of the ways that the ECM responds to the knock sensors picking up the sounds of knocking. It can also retard the the point where the spark plugs fire.

 

Thanks!

 

I goofed-
See post above

 

Not sure if this helps out too many people, but for what's it worth, I just checked the compression on my 2001 XKR 4.0 SC and it is at 180+ psi. I made about 10 turns of the engine with the throttle WFO.

 

Marc,

How many miles are on your XKR?

 

180 PSI is good and yes you are looking for readings close to each other. Two cylinders that are beside each other and having a real low reading like 60 PSI means that the head gasket is probably blown between those cylinders. Low reading can be a valve problem, head gasket leak, or a ring wear or broken rings problem. Squirt some oil into the cylinder and if the reading comes up it will be worn rings. If it doesn't it could be broken rings (seen this one), head gasket, or bad valves. At top dead center on compression stroke feeding compressed air in will let you hear where the the leak is. You have to look for bubbles in the radiator, too.

The theoretical is nice for sealed cylinders, but it is hard to use for engines. Valve overlap is around top dead center on the exhaust stroke and at the start of the intake stroke so that isn't a factor in compression tests. Also it is hard to figure in, just what is the effective compression ratio due to the fact that the intake valve is not closed at bottom dead center. The valve doesn't fully close until later in the compression stroke (this is where the PSI is made)and that depends on the cam timing, duration and valve lift. In the same engine changing the cam from a stock camshaft to a full race cam will drop the PSI reading, but the horsepower will be a lot more.

 

Sinister:
Remember that an XKR SC has a lower compression ratio than your engine- It is 9 to 1 as I recall.

 

Class time.

This will be about "small block V-8 engines". That is 5.7L/350 CID or smaller. Figure 0.7125L/43.75 CID per cylinder on other engines. Most Jaguars will have a "small block engine." If you have a big block; take away 10 to 15 PSI from the following.

If your compression test is done correctly(wide open throttle and not too many times around)then 160 PSI and under; we are talking 87 octane gas. Go above that up to around 180 PSI and you are looking at 91 octane gas. Now there have been a few of the super tuner guys that have hit over 200 PSI, but that isn't really a street machine.

There are other factors such as two spark plugs on opposite sides of the combustion chamber, center mounted spark plugs, small combustion chambers, direct cylinder injection and water injection that can cut the spark knock.

Super charged engines will have a lower compression ratio and thus a lower PSI reading. Inter coolers can raise the C/R some and so can dropping inlet air temps etc.

 

78,112 miles.

I brough it in to a shop for a random misfire. These guys specialize in exotic cars and the owner is an ex Jag mechanic. They came up with 16 codes. They changed the fuel filter and found what I would call customary amount of water in the filter. (We can thank the government for the ethanol in our gas for that.) They told me my spark plugs were fouled, and I had the best belly laugh in the longest time. They then told me it was because I had 4 bad ignition coils. Now I wasn't laughing. They charge 100 bucks an hour labor. "Do you see me wearing a skirt fella?"
I tried to explain to them that sometimes an "ignition" can fire a spark plug out of the engine, but not when installed under compression. My XKR runs like a bat outa hell at WFO, there is NO MISFIRE at WFO when the compression is at it's highest so that elimates coils, plugs, compression injectors etc. , but this misfire and the fail safe mode...It came about while slow cruising around town in a school zone.
"I'll change my own plugs mister, but I'm telling you this is a SENSOR PROBLEM." This issue is only when the engine is in "closed loop". I don't think he grasped the concept, so I took the car home.
I changed all 8 plugs and done a compression check, because I suspect that "IF" there is an issue with the camshafts, it MIGHT give me a clue by looking at the readings from bank to bank. Maybe.
BTW, I was right about the plugs. They were normal in every way. 40 bucks wasted. Could have been $200.
I'm convinced that one of the knock sensors is bad, ($50) but I got so much other stuff going on right now, that I haven't the time to check the wiring or even find the dam things to switch them out to see if the codes change before I spend whatever more to rid myself of this gremlin. Any of you guys can tell me where to find those knock sensors? The code is saying misfire on 5, 6, 7, & 8. The vaseline is still dripping from my butt after dealing with those guys. Sometimes if you want it done right, you GOT to do it yourself, or pay the price in towing, downtime, money and headaches.

 

Marc:
I do noit believe there can be a connection between low speed misfires and Knock sensor. But, Have ago- they are located at the top side of the heads between the second and third inlet ports, I believe. You can Id them, they are the only sensors up there.

 

There was a lot of discussion in the low coolant thread about misfires because the knock sensors plug connectors were getting wet. (It hasn't rained here in over a month.) It was told that when the knock is detected it causes the ECU to retard the timing on that cyl bank. It was speculated that even the valve timing might also be affected. My engine is acting like it's way over cammed. Even the exhaust note is much louder but there is nothing wrong with the exhaust.
I do not see the knock sensors. One would have to pull off the intercoolers to get to the intake side of the heads. I can barely see the injector wiring under there.
No over temp has been detected in the cats. My Mustang friend even commented, "your car sounds like mine does now", (since he installed some big thumper cams). He thought my cam chain tensioners are shot. (please please not the dreaded cam tensioners), I don't hear much noise in the cam cases. I understand there is an advance/retard unit on the intake cams. That would be my next suspicion.

I have stopped driving the car. The exhaust odor is not right and the car is not smooth as it usually was. My Jag dealer service rep told me to disconnect the battery for half an hour to clear all the codes and try again. Is he right?

Anybody have a procedure outlined for the knock sensor R&R or trouble shooting those knock sensors on the supercharged 4.0? My idea was to buy one new one and swap it on bank 1, then swap the used one to bank 2 if that doesn't cure the situation.
Anybody have a simple procedure on trouble shooting the camshaft advance unit on the supercharged 4.0?

 

Marc,

I'm chasing a random miss also. Mine is at part throttle acceleration and very occasionally at idle. Oh yes and it's very intermittent.

For example, I drove the car 408 miles last Thursday without an issue until the last 20 miles. All of the driving was freeway then all of the sudden it starts acting up for no reason.

The stumble happens mainly under part throttle acceleration when the engine is "pulling the load". I've plugged the OBDII reader in so many times I'm going to wear out the connector. No codes at all. I'm at my wits end and am about to start changing parts (which I deplore) just to see if the issue goes away.

I've been through all of the diagnosis and at this point all I can think of is it's not a "Miss" but a Lean Stumble instead. I'm going to get my injectors cleaned in the next few days and see if that helps.

 

Back in the dim times we watched an oscilliscope to see what is going on.

With a scan tool that reads "Live Data" you should be watching what the spark timing is doing, fuel rail actions, and be sure and watch what the Throttle Position Sensor is doing. The TPS should move smoothly with the accelerator pedal. It or the pedal may be headed for the last round up, but not throwing a code yet. Look and make sure what the ECM sees is correct. Inlet Air Temp should be right as an example.

If you don't have it get a copy of Haynes Techbook OBD-II & Electronic Engine Management Systems. It will help you test all this swell OBD-II stuff. Remember, a lot of the stuff on a Jag isn't made by Jag, they bought it 'cause it's cheaper that way.

 

The old compression pressure rules of thumb should be taken with a pinch of salt: The NA AJV8 engines have variable cam phasing and under idle and off idle conditions the intake cam timing is fully retarded to minimise valve overlap. The NA engine has a geometric CR of 10.75:1
The S/C engine uses a geometric CR of 9:1 but has no variable cam phasing and a MOP (maximum opening point) of around 120 at idle/off idle and an earlier IVO. This may give the illusion of a higher compression pressure.

 

The AJV8 has had cylinder specific knock control for quite a while and it doesn't retard the ignition value for just one bank.
What was observed during engine development, however, was the knock sensor on cylinder 2 B picking up noise and retarding too much (8 degrees). The AJV8 really should have about 4 knock sensors and not just 2.

 

Injectors do cause low speed misfires too. I experienced this on a late model chevy 3/4 ton.

Somehow coal dust was introduced into the fuel tank and it made it past the fuel filter. The driver had to replace the fuel filter 3 or 4 times on the trip home from Virginia to So Louisiana.

The loss of power and the low speed shake prompted me to isolate which cylinders were the culprit. We then sent the truck to a major brand repair center. They changed 2 of the ignition coils and called it good. I was not happy since I told them it was a fuel issue. I kept the old coils to use for future trouble shooting on our fleet. I pulled the fuel rail and found it full of coal dust. I then replaced 2 of the injectors to cure the misfire.

Misfires can be hard to describe. One would have to trust their gut and use the process of elimation to pinpoint the causes. For instance, my XKR has it's full power @WFO so this indicates at least the coils and the plugs would be good.

 

Thanks everyone. Very good information here.

This intermittent stuff with my car is driving me nuts and I'm trying to rule out things one at a time. Between this intermittent stumble and an intermittent coolant leak, I'm finding that understanding women is actually easier than figuring this car out.

As far as scopes and a reader with live data I don't have them. Even if I did, this is intermittent enough that I'd have to keep them on and plugged in at all times and be ready to read them at a moments notice. Once I've had enough, I'll let the dealer do that.

I saw on another thread that the MAF could be a suspect for a stumble at idle. Since I get those on occasion also, I'll poke at that tomorrow and see what happens.

 

MAF is clean and checks ok at this time.

It's still to darn cold to be outside working on the car so I'll just keep driving it as is. Like I said it's very intermittent so it's not a problem in the daily commute.