I am again helping a friend get his XJR up and running after engine swap. Car is driving fine but still feel like something is a bit off. Car is not as fast as my SVP and I am carrying a bit more weight.

I tapped into the boost line last night and only hitting about 9psi.
max.
Everything seems to run fine at idle, low throttle and even WOT. Just doesnt seem to push anything over 9lbs. Also getting low teen MPGs

Here is what we tested and know:
Not pulling any codes, no recorded misfires
Fuel trims look good
Bypass valve working. - Atleast can see valve arm moving 90 degrees.
Tested for vacuum leaks multiple times and found nothing.
Fuel pressure is 38psi at idle, goes up to 50+ at WOT high rpm.

No visual signs of boost leak in hoses or gasket. I doubt you could hear a few psi bleeding off over the SC noise. No signs of belt slipping at high rpm.
No signs of ignition problems like hesitation.

I could use some ideas on where to go from here. Leading culprit right now is the cats. I have never done it but another guy can tap pressure gauge above o2 to measure pressure on both sides. A few months ago bolting up exhaust the cats looked clean from the underside. Maybe they are clogged up top....

Outside of cat convertor:
1. Double check Throttle cable
2. Better way to find small leak in gasket or hose?
3. Could there be ignition issue only at high rpm, like a weak coil?
4. I did not compare my plugs with his but friend mentioned the plugs he used were a step colder than stock.
5. Not sure how well intercooler pump is working, IAT2 doesn't record.

Could maybe 3psi of bleed off come from a number of issues? Small leak in valve covers, small gasket leak, cold plugs, weak ignition, high IAT2, etc.

Does this sound about right? Am I missing something? It is amazing the difference the last 3 psi makes when driving. My heavier car feels much faster.

 

Is the TPS properly adjusted? ....also, have you cleaned the MAF? Mine was getting sluggish, I too thought the bypass. But as you state, the actuation seemed fine. Cleaned the MAF, it gained a bit, but still not to performance level...replaced MAF....put down some 11's.

 

what brand/model number MAF did you use? I see on eBay they run from $35 to $335 depending on generic, Cardone, Denso etc.

 

I got mine off Amazon...https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1

 

I've racked my brain over this. I'm pulling boost from right after the SC. Except for top-plate gasket there is no place it could leak. Leaves me to assume it is starved for air.
First check is the throttle cable and TPS. Even though I am flooring it, maybe car doesnt read it.
If I dont see anything there I may push to pull the SC, rebuild, new gaskets and clean up. I'm wondering if it's a case of "sum of all parts". Maybe weak coils/ignition, cold plugs, a spitting injector and possible small vacuum leaks like oil cap, worn hoses could all add up to loss in power.

 

I think you are way over thinking this. "Boost" is a function of the output of the S/C's rotors turning. In other words if your RPM is at 3000 then your S/C would be spinning at a set amount based on its pulleys and put out a certain amount of boost for said RPM, to increase "boost" one would put on a smaller upper pulley so now at 3000 RPM you have a higher s/c rotor spin rate and thus more boost. In other words a bad spark plug or fuel injector is not going to change the air volume output at the same RPM. There's either a clearance issue (too much) between the rotors or a leak at the elbows or top plate or an intake restriction (how's the airfilter, snorkel?) the s/c itself is pretty simplistic in design, you can get away with pulling up the top plate and checking the rotor condition. Once lifted remove the belt and spin it through and make sure the rotor coating isn't flaking off or there's no sign of scoring/rubbing. Also check where your tensioner is at, it'll move as the belt stretches and it is imperative to have a good belt with the proper tension on it.

 

I rotated mine, when I had it off for tensioners and piping, to check it with a socket (8mm if I recall) on a drill. In your case, just drop the belt, pop the cover off the outlet (check the seal) and spin it with the drill to get the rpm's up. I was using a cordless 12v with a diode stop, so I had to pull the drill back off as it was spinning it or it would stop almost immediately. You won't have to have a hot motor to deal with or the noise from it to detect noises.

 

Trust me one this one (addictedtoboost knows) trying to detect a noise in the s/c with a drill and the lid off in most cases is pointless (unless its bearings and even then it may not show up with zero resistance) . I had two units making horrid noises and in both cases they would perform "noiselessly" when spun with a drill. The coupler noise or gear train/lash noise usually won't show up with a drill, the reason they make noise at idle is due to absorbed pulses from the crank and back pressure at idle especially given the small size of the Jaguar bypass valve. I even had one rebuilt and it would spin perfectly noiseless after rebuild with a drill but sound horrid on the car, changed to a urethane coupler solved the noise issue on that one.

 

Good to know JT...Thanks...do you know if using a motor stethoscope would work using the drill?

 

I had no luck with that either. The guy who rebuilt my S/C threw his arms in the air, refunded my labor costs an vowed to not do Jaguar S/C's in the future. The m112 is a popular s/c but the common thing in Jaguars from what I can tell is that bypass will not bypass all the boost at idle and they are very sensitive to the coupler used and gear drive backlash at idle. I bought 3 different couplers to try out because when I sent mine back to him for noise after the rebuild he put in a different coupler and it made matters worse! (No sound on his end on the bench with a drill) I went with the white urethane one as it was a little "plyable"him and I both ran it full song on a drill on the bench and it made zero noise, on the car it was a whole different story.

 

Col. S., have you done a compression test?

 

+1
I think the s/c is suspect. As stated, airflow is airflow.
If the rotors are nor set up properly or worn, this could cause low airflow.
The rotors really cannot be checked visually. They must be removed and inspected in very specific conditions. Kinda like the fan blades on a jet engine.
If the tolerances are not set right, you will not get the draw that the sc is supposed to breath in.

Just to ask, is the belt system set up correctly?
Are the gears that turn the rotors engaged properly?

Also, what is the pressure at the exhaust?
A clogged cat might also cause this.

 

Have you checked the crank pulley/damper is not slipping ???

 

9 psi ... have you baselined the measuring instrument
against another vehicle?

I see crank pulley and belt slippage have already been
mentioned.

Is it possible to jam the bypass closed during testing?

Couplers, etc. may be irrelevant because if it turns, it turns.
Maybe noisily, but it will still turn and build boost.

 

If all else fails, maybe it really is the s/c leaking air? I've just been reading up a bunch on superchargers and like others have said, if the rotors aren't exactly where they need to be then air will leak past and you'll lose some boost.. though I'm not sure how significant it would have to be to lose 2-3psi.

Maybe when you pull the V8 plate off you can try moving the rotors "up/down" orthogonal to axis of rotation and check for play there, if the rear needle bearings are going bad. If it's real bad you might be able to see scratching on the rotors or inside the case, but that's kind of guessing.

Before messing around with that though, comparing against a known well-running car is the place to start as plums said, with the same rpm and conditions.

JTsmks, which coupler did you end up using? Is that the "nylon oil-filled" green coupler seen around on ebay? In other words, not the spring-loaded OEM coupler?

 

I want to be clear, I was not suggesting a coupler would cause low boost, just got sidetracked a little bit. I've heard, injectors, plugs, cats etc. if one is measuring "boost" at a specific RPM none of those items are going to make the "boost" measured at a specific RPM go down, boost is a function of air in and air out. Your cars general "power" has to do with plugs, cats, ecu etc.


To answer the other question, I went with this white urethane coupler, the green and grey ones were hard as rocks and made lots of racket at idle. I found the white one on eBay and am very happy with it.

 

Col. Sandurz,

Did you ever happen to solve this issue, any suggestions on what it caused?
I am in the exact same situation, even so far at that it is a friend's car .
Think you are still around, right?

Appreciated,

Eric

 

I cannot say for sure it was solved.

Top end of car was taken off. He was convinced it was the bypass valve so everything came off the top, SC rebuilt, new hoses, cleaned and all put back together. I was not there when the boost test was done after all the work and i know it was done with a different instrument.

Between not finding an obvious fault and using a different set of gauges to test again it's hard to tell what was off. Boost was back after rebuild.

 

Thanks, that is useful info, appreciated the feedback.