New Engine can't idle

XJ
1

Hi, i’m Francesco Sapienza from italy.
I bought an xj6 series 3 4.2 a while ago, it had a blocked engine, because cyl 1 was stuck due to corrosion made in the years the car stayed unused.
So i rebuilt the engine (bore, new pistons, bearings, valves, guides, face milling, ecc)
Now the engine is on the car, and everything is fully assembled, the only problem i’m having is:
The engine once started revs to 1200-1300 as it should, it seems smooth, but when the aav and the start injector closes (as they should) the engine wants to go to 800/900 revs (as it should), but while rpm lowers to idle it starts to misfire, so it can’t hold idle and goes to rough idle till stalling.
If i push throttle a moment it revs up, but if i keep pushed at a certain value it dies the same.

Now: I’d exclude the fuel line, because the pump is new, the filter too, the injectors had been refurbished just today, and if i remove a tube from the filter or somewhere else, the fuel that comes out is clean.

I’d exclude the ignition timing too, because turning the distributor didn’t change anything and I assembled it with the same orientation it had before disassembly.

The throttle body is registered to it’s correct opening

The maf looks like was not opened

The temp sensor nearest to firewall works, because if i disconnect it, the problems goes worse

Tested quite all the vacuum line but seems to be ok

I’ve done a lot of tests but nothing changed anything.

The only things i’ve not changed or refurbished are the coil and the distributor cap.

May those two components be the cause? Consider that when the engine manages to rev up somehow it seems smooth.

It only has problem at steady throttle, idle or not.

2

You obviously know what you are doing, Francesco - and very clearly explained…

Have you actually driven the car on the road to confirm that problems are indeed confined to idle? And ‘steady throttle’ is somewhat ambiguous - with higher throttle settings the engine would be expected to run (how high have you tried?)

It’s a characteristic of the xk engine that it is dead smooth at higher rpms even with some major issues - only noticeable in idle.

You are excluding some measurable qualities without testing; only fuel pressure tests will reveal whether there is adequate fuel supply. Likewise; ignition timing should be checked to spec, not assumed - it’s a safeguard…

You do not mention engine temp related to the drop in rpms. Obviously(?) you have not been able to set idle properly; it must be set with the engine fully hot and idling. If the idle is set incorrectly/too low, the engine may misfire and die as the AAV closes before the engine is properly warmed up. The AAV is electrically heated to close faster than the engine temp is increasing…

Otherwise, the symptoms described may indeed relate to a vacuum leak - a major culprit may be the ducting between AFM and throttle body. Checked?

One test is to remove fuel filter and, with engine ‘idling’, gently push in the AFM flap. This will increase fueling, but is veeery sensitive - if the idle increases; it indicates a vacuum leak…

While coil and dist lid is important for ignition; either should be independent of rpms - but the latter may relate to ign sequence. Has ign sequence been checked and absolutely verified; crossed plug leads is a possibility.

Another possibility is the ign amp; some exotic faults sometimes debilitate it - at low rpms or temp issues.

Lastly; while assuming the reassembly was carried out with due diligence; have you run a compression test - which is unambiguous. And is the valve timing to specs? Just asking as part of somewhat odd symptoms…:slight_smile:

Frank
xj6 85 Sov Europe (UK/NZ)

3

Hi Francesco,

You’ll get it running! For some reason I’m thinking it’s too lean.

It’s been a while since I had a Series 3, I now have a Series 1. I too had to replace pistons in the seized engine. Soon after that S1 had a clogged fuel filter and it would start and idle perfectly, but at higher speeds it would stumble - unless I accelerated and the full load enrichment added more fuel. That’s my approach. You get enough fuel from the pump, as it is not getting worse with increasing load…

I believe the L-Jetronic has a similar full load enrichment. When you accelerate, it will inject more fuel as the vacuum drops which is sensed by the vacuum switch near the throttle body. At any stationary speed the engine runs lean, but while you speed it up there is enrichment that makes it run nicer. While cold it runs rich so until the temperature goes up there is enough fuel.

If you add ether or brake cleaner, etc. at idle, does it run better? Then you know it’s lean and not ignition, etc.

It could be a vacuum leak. I’d recommend a smoke machine, you can build one with a jar and a bit of hose. Drill two holes in the cap and push the hoses in. Tape the intake off, light some paper, close the jar and blow compressed air through it into the intake. Smoke will show the smallest leaks.

I’d exclude fuel system, ignition sequence, throttle settings and compression. Valve timing will show up more at slower speeds but it quickly smooths out at higher rpm so I’d assume that you put the engine back together correctly. From your description I’d say you didd good work and that there’s nothing wrong mechanically.

Good luck!
David

4

Thank you frank, it is impossible to test the car on the road, because the engine stalls too easily.

The temp it’s around 50° because the engine can’t warm up well.

The hose connecting the maf to the elbow looks like it’s good.

The throttle body was registered to the correct opening said by jaguar (0.002 ins if i remember well), because it’s impossible to register the idle, always because the engine dies

For now i can’t do anything on the car till the 20th of February

But I’ll check what you said when I’ll be able:
Fuel pressure, timing (with strobo), manually moving the maf and so on. Thank you very much for the help.

5

Yeah, I didn’t think about it but you’re right, the engine works well with throttle taps, so it must be due to the enrichment.

I didn’t find any vacuum leak, but i didn’t try with smoke, thank you David for your help. At this point i think i will re control even mechanical assembly and mostly important i will control ignition timing.

With the stroboscopic light what is the correct value i should read if everything is correct?

6

My experience with my series 3 is that mystery misfires can arise from poor earth connections - carefully check the engine block to body earth strap and everything else that should have an earth - and dirt in the fuel from corrosion in the fuel tanks. If the car has been standing outdoors for long periods, rain water and every kind of rubbish can enter the tanks and cause problems.

1 Like
7

34? Maximum, 10° static, it is possible that the damper moves and the timing scale is off. Measure TDC through the frontmost spark plug and check if it matches up with the 0° mark on the damper. But I don’t think it’s your timing.

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[quote=“Cicciosap, post:4, topic:430075, full:true”]
The temp it’s around 50° because the engine can’t warm up well.
[unquote]

Idling, the engine needs at least a quarter of an hour to get warm, Francesco - stops before then…

Sounds like lean stalling, as David says - not enough power to idle below 800 rpms. Check resistance over the two-prong coolant temp sensor - at 50C the resistance should be 840 ohms. The higher the resistance the fatter the mixture - disconnecting the sensor, the mixture goes very fat, and a warm engine will misfire/die. The symptoms of too lean and too fat are very similar…

Fattening the mixture by gently pushing on the AFM flap may clarify if ‘too lean’ is the problem - though not ‘why’. Lost ignition is detectable by watching the tacho as the engine dies; if the tacho drops to ‘0’ while the engine is turning signifies ignition lost - usually ign amp…

Another diagnostic tool is to connect a vacuum gauge to the fuel regulator hose. 18" Hg steady is ‘good’, too low indicates lack power - again not ‘why’. ‘Erratic’ readings can be interpreted to specific faults…

Again, with any suspected fuel problem, fuel pressure should be checked…

Frank
xj6 85 Sov Europe (UK/NZ)

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Thank you all for your help, you are giving me a lot of good advices.

As i said, when ill’be at home for the 20th il look for what you said.
The temp sensor’s resistance and the timing so thanks for the correct values.

The only thing i’m pretty sure is that the grounding is good, i’ve checked multiple times.

The injectors ground is not in common with the frame, but i think it might be normal

10

The injectors are 12v on both leads, one lead is grounded by the ECU for firing. The other is powered through the 2PR resistor pack on the body underneath the air filter. The ECU gets its grounds from the many wires bolted to the rearmost bolt on the water rail.

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So injectors shouldn’t make continuity with the frame, right?

12

While they fire, but with ignition on you see voltage on both pins and you can hear them click if they work. Search the forum for bullet connector.

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Hi Francesco!

I had a similar problem some two years ago with my XJ12 III HE. Back then, a faulty ECU was to blame, it let the engine run way too rich. I documented the issue in the post below.

Of course, there are main differences between the engine setup of the XJ6 and the XJ12, but maybe running rich could be your problem as well? This would also explain why opening the throttle slightly helps the engine stay alive for a few more moments.

What do the spark plugs look like?

Best,
Florian

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Hi Francesco!

great to have you here. I feel like I have seen this movie before. My engine (xj6 S3) was not overhauled but I had confirmed that it was working fine before I took it out of a parts car. Yet after installing it into my car I had months of trouble before I could get her to run. The car would start and then run fine if you rev it but die at idle. Also for some reason while disconnecting the AFM it would somehow idle ok… You will eventually get it running. It just takes some doing. Trust what others here are saying. They have helped me immensely.

You may want to try disconnecting the AFM and also trick the temperature sender to let the ECU think the engine is dead cold. You should get more fuel that way.

To this day I have a pressure gauge installed onto the cold start injector. This is very useful for troubleshooting. You will immediately see if there is a pressure fuel related issue.

With the XK engine design there are a lot of ways to get a vacuum leak into the system. I suggest to take the 10 minutes to build this DIY smoke machine. Just a cucumber glass. With 2 hoses. The one should fit onto your air compressor source. Dial down the pressure to very low. And then use packing paper, you know the brown stuff that burns well. With the lid open burn it down half way and then just close the lid and start pushing air through the glass. If it is not air tight use some masking tape to stop the glass from leaking. Put the other end into the intake. Any way that is convenient really. You can confirm that the system is working if you leave the Oil refill on the left side cam cover open. Smoke will come out of there. If that is the case your system is working great. It makes finding leaks very simple!

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For me in the end it was actually a combination of things.
I had massive leaks even tough I resembled everything correctly. It was actually a bolt missing in the intake bridge that introduced a pretty bad leak…
Secondly I had confirmed I had spark but It was not enough spark. Also don’t rule out things just because they are brand new! These days sometimes even new is just dead on arrival! Just my experience. Once I changed the rotor cap and pins the engine ran.

You also need to start driving the engine. Maybe this is different with an overhauled engine but mine just needed exercise and a few of the problems like hesitation went away. I still have a misfire at idle I am chasing down. Like Frank said the XK will run great at power even with lots of issues that only show at idle.

Don’t get discouraged you will get it right, it just may take some time.

15

You will.

The smoke machine was Max’ idea, I admit I didn’t believe it would work at the time but it was such a great way to show even the smallest leaks that I’m now sold and recommend his device for all kinds of leaks. I don’t think the engine is running rich.

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I would agree. My experience is that the XK can run super rich and still run ok.

1 Like
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Hi everyone.
I’m back home and now i can try everything you suggested.
The first thing i did was the simplest one:
I measured the resistance of the thermistor, with the engine completely cold it says 4kohm, which are too much in my opinion, I’ll find a way to warm the engine up to 50°C and re measure the resistance, but now it i’m starting to think it’s a thermistor’s fault

18

I couldn’t warm up the engine because it started only two times and then it wouldn’t even start.
I checked sparks, and they looked very poor, i opened my distributor and the cap broke in my hands… so for sure now i’ll have to buy the rotor and the cap,
Do you suggest me to buy even the coil?
I’ll get even the stroboscopic light and the dial indicator to do the right ignition timing.
And only then i’ll be sure if it is spark issue or not.
This problem is really confusing me, and the fact that i’m often far from home makes everything even worse.
Maybe I’ll change even the thermistor, just to be sure…
I really don’t know why this engine won’t start…

19

Why didn’t you try the bullet connector?

New rotor and cap is a good idea, they were brittle already and it’s good you found out in a convenient place.

No need for a dial indicator and timing light. Screwdriver is enough.

If you say weak spark, why don’t you figure that out first (new cap) and then test if it’s better then come back. One thing after another!

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What are you specifically saying with bullet connector? Which one of them?
Because bullet connector it’s a kind of electrical connector in general, what of them are you referring to?

I would like to work one thing after another, but sadly I’m a bit on a rush, and I even need to keep costs as low as i can.