Hello all,

I have been working for several weeks now trying to get a freshly rebuilt 4.2 in my 1986 XJ6 to run. I went through several iterations of troubleshooting, and finally discovered a couple of weeks ago that I had the ignition timing off 180 degrees. Still not sure how it happened - I’m very meticulous and followed all the instructions, although I’ll admit I’m not an expert at these jobs by any means.

Up until the point I discovered the timing error I couldn’t even get it to sputter. Upon fixing the timing it would sputter a bit but wouldn’t run. After some visual fine-tuning of the ignition timing I indeed finally got it to run, but it’s running very poorly, basically on two cylinders, #1 and #6! Honestly I’m surprised it runs at all. But I’ve tried to diagnose the hell out of this thing and now I’m pretty stuck - I can’t find any other problems. Here’s all that I’ve tried and verified while attempting to diagnose:

Fuel system:
- Fuel pressure is consistent (about 32 lbs) as I have a gauge installed and can monitor. When fuel pump shuts off pressure holds.
- I removed and re-cleaned all the injectors and confirmed spray pattern is good.
- Confirmed injectors are spraying after re-install by using a test tool - fuel pressure drops as I cause each one to pulse manually.
- Confirmed injectors are receiving electrical pulses by using a noid light to check at each connector. This is how I found that cylinders 1 and 6 are the main working ones because there was no evident effect on how the engine was running when disconnecting the injectors on any of the others.
- Fuel is fresh and fuel tanks are new (the old ones were awful)

Ignition system:
- Replaced all wires and plugs. Double checked gaps and they are to spec.
- Replaced coil
- I have a friend with a similar year running XJ6 and he let me borrow his distributor, cap, and rotor to make sure there was no difference when using his parts, and there was none.
- I installed a new NAPA ignition module in the amplifier box.
- Spark seems to be fine - I can use an inductive timing light and move it from wire to wire while it’s running (badly) and they all cause flashing
- Ignition timing per timing light is within a couple of degrees of where it should be (I believe it’s currently reading about 12 degrees BTDC)

Mechanical:
- did a compression test on all cylinders, and it’s a bit low (averaging 90 lbs) but these are brand new rings and honed cylinders that haven’t seated yet. However the fact I’ve at least got compression tells me valves are doing their job and probably close to correct timing
- pulled valve covers and double-checked valve timing. It’s still looking to be set perfectly - I bought the cam alignment tool and had set it by the book during assembly. Timing chains seem properly tensioned, and I turned the engine over manually and watched, all looks and feels good. Also checked tappet clearances and they’re in spec.
- Side note, the engine is pumping oil nicely as there was oil pooled in the head and it shows pressure in the dash gauge while it runs.

Electrical:
- Re-cleaned and tightened ground points, both the big one at the trans/engine block bolt and the small ones at the rear of the head. These seem fine.
- Earlier in troubleshooting I noticed the negative battery cable was heating up when cranking the engine, and I ended up replacing both battery leads and that's when I cleaned up the ground points. No further hot cables (it cranks stronger too). But this was before I even got it running at all.

I’m baffled, and if anyone has thoughts they would be most welcome. If I’ve got good spark, and fuel, and mechanically everything is sound, how in the hell are only SOME CYLINDERS firing?

Thanks in advance for any help or ideas.

 

Hi its very frustrating trying to get these Motors going. Have you checked all your earth points? there is on on top of the head at the rear of the motor probably on the passengers side in your part of the world and there is another on the bottom of the block, but not sure where. I know some members have added or replaced this one to make sure that all earth points are good. Not sure that this will be of any help but cant help to check
Good Luck
Con

 

How do the plugs look on cyls 2345, wet or dry? What does the spark look like on plugs 2345 when testing individually? Never assume when troubleshooting...

 

Thanks for the comments! Actually I did clean and check the grounds, both the ones at the head and the big one between the car body and the block. I will check again but I believe these are in good shape. I will add it to the list of things done already in my original post.

 

I have pulled plugs a couple of times since starting this troubleshooting. #1 and 6 look normal, still clean, and #2-5 are a bit darker, like they're accumulating a little bit of soot. But it's very light since I have only run it in short bursts. But pretty dry, not coated in oil or fuel.

To your point about spark, I will pull a couple of the middle ones and visually check to see what it looks like. Will post a pic if I can.

 

buy one of these and work your way back to

the dizzy. cheap and fast
geneo

 

Thank you for the suggestion - actually I have one, and I just went and double checked. All cylinders are lighting this up pretty well.

Going to triple-check injectors and their circuits now - it occurred to me that perhaps despite them working well through my tester the car may be sending them less current so perhaps the spray is weak. Even though the noid light was flashing at each connector...

 

Even on a rebuilt engine that seems really low. What was the highest value and what was the lowest value?

For comparison, here is the compression test on a freshly rebuilt V6 I put together a few years ago. The engine is on the test stand and had freshly honed cylinder walls and new pistons/rings. I know it's a different engine but I'm surprised your compression numbers are as low as they are.

 

It sounds to me like the HT leads are in the wrong order. Firing order is 1-5-3-6-2-4 with distributor turning anti-clockwise. So if the HT leads are in reverse order, you would get firing on 1 & 6 only but not the other cylinders

 

YES!! You are my hero! And I can’t believe what an idiot I am Even though I knew the distributor goes in reverse I was still connecting the wires clockwise. It fired up and sounded so beautiful… and then died after about 5 seconds. But I’m so happy to be past this issue I don’t care. Thank you so much!

 

So as I said, very glad to have the engine be firing on all cylinders now, but it doesn't run any more than a few seconds. I have the fuel pressure gauge still hooked up and the fuel pressure doesn't drop. I assume the computer is killing the fuel injectors for some reason, since spark isn't controlled directly by the computer (correct?). The odd thing is that when the spark plug wires were wrong and it was barely running it would keep going indefinitely - no cutoff. What is the most likely cause of this?

So far I have verified that it's not the ignition switch, as jumping battery+ directly to the coil has no effect. And as noted fuel pump seems to continue running, at least until it dies and gets cut off when the flow meter flap closes.

 

Hello Incipheus, Can you try one thing just for giggles? Disconnect Temperature sensor plug connection and bridge with paper clip please, Then try to start engine. What happens? Second test, Engine off but key to on, press the flap in the airflow meter, Can you hear the fuel pump run? What happens?

Larry Louton

 

Just to add my tuppence worth.....
There is no "computer" in the EFI system on these cars, it is an analogue system with no programming at all.
One defect that not a lot of people know about, (and I found out after laying a pile in my pants !), is that the whole fuel supply side is totally dependent on a couple of very small contacts in the airflow meter. These contacts are closed when the AFM flap moves on engine start, and allow volts to continue passing to the fuel pump. During cranking, these contacts are bypassed and the fuel pump supplied directly. Once the engine starts, and the ignition switch is returned to "Run", the volts supply to the fuel pump goes via these contacts. Over time, (now decades, of course !!), these contacts lose their springiness and can fail to make a good contact intermittently. I had to remove the top of the AFM, and "bend" the contacts to make sure of a good contact. It's crap design, I'm afraid, and put there in case of accidents to make sure that when the engine stopped, so did the fuel pump.

 

Been out of town on business for a few days but will try this today. Just to be clear, which temp sensor are you referring to?

I will also confirm nothing has changed, but at least relatively recently when the ignition switch is in on position and I press the air flow flap slightly, fuel pump runs. Pressure holds at about 32psi, and doesn’t drop even when the pump stops.

 

I actually cracked open the cover of this meter, because I know it’s a common source of problems. Of course it could be bad, but it didn’t have any corrosion inside and contacts looked to be in good shape. As noted above the fuel pump switching action worked fine.

 

It could be that the motor is running only on the cold start injector (independent from the fuel injector circuit) which may only spray a few seconds on cold start, not sure on L Jetronic but is crank position sensor signal required to fire the injectors?

 

Hi Larry,

I had a chance to try what you suggested this morning. If I bridge at the temp sensor connector (I assume you are referring to the sensor that is second from the back on the water rail above the intake, lined up between cylinders 2 and 4), the car won’t run at all. It sputters slightly but will not run. The same thing happens if I leave the connector disconnected but without the short. If I reconnect the sensor it goes back to running for a few seconds and quitting. I notice at ambient temperatures (a warm day, but cold engine) the resistance of the sensor is coming at around 1950 ohms. Is that about what it should be?

I believe my cold start injector isn’t working at all - I tested it during engine reinstallation and it wouldn’t open using a tester, but I put it in place anyway for now. Shouldn’t be necessary anyway since I’m in Arizona and it doesn’t get very cold here. Regardless, I disconnected the cold start injector and the engine fires up for several seconds as before to eliminate this possibility.

 

In today’s tests, I borrowed an air flow meter and an ECU from a friend with a running XJ6, and no change. I’m fairly convinced this car is taunting me for the prior error.

 

Confirmed that fuel pressure good, again. I put mild restriction on the return line so it was up above 40psi, and this is maintained until after the engine dies. No effect.

Checked voltage at the fuel injectors, and matches battery voltage pretty well (12.3v). Using a noid light at the connector I can see that the flashing stops just before the engine shuts down. It seems pretty obvious that pulses are being cut off by the ECU. BUT WHY? And why didn’t it cut off similarly when the engine was barely running? I’m convinced there is a key here.

 

I think it would he worth your while spending some time checking the fuel injection system harness wiring.
I would meter out all the connections from the ECU to the engine, and in particular to the injectors and AFM. It is not unusual after many years of use to get fractures in the wires, especially in the engine bay area which gets very hot.
When I bought my Series 3, it had been out of use for several years and did not run. Among the many problems I found were some fractures in the wires to the AFM. This kind of fault can be intermittent as the wire can make and break with slight movement or temperature change.
To check the wiring, you will need a long piece of wire as an extension so that you can connect a multimeter probe at either end of the car. For example from Pin 1 of ECU to the ignition coil. Give the wires a wiggle as you check them for the aforementioned reason. Work through all the pins of the ECU connector plug one by one.

Another check would be to see if you still have 12V at the injectors when the engine cuts out. If not, suspect the Main Relay which supplies them with 12V (via the power resistors) as well as the 12V main feed to the ECU and AFM.