1992 5.3 Marelli - Stall after warming up and odor

(Geekay) #1

My 92 has 53k miles on it and needs to be driven more. About 2 months ago I took the car for a short ride and I barely got home as the car was trying to stall out. I do have heat from both exhausts.

I looked under the hood and I saw arcing on one of the plug wires.

So I’ve cleaned the vee, replaced plugs, coils, leads & fuel injector connectors and made some minor repairs to the loom, which is in overall good condition. I raised it and wrapped it with special heat tape too.I have a replacement Marelli rotor but I did not see anything wrong with the existing one so I have not replaced it yet. A few months ago I replaced the CPS.

I also cleaned out the mouse crap.

After completing the work the car cranks fine but after a short time…perhaps a few minutes, it starts to stall as though it’s starved of gas.

To compound things I have a strange odor inside the car that smells like a strange burning smell but I cannot see any issues under the hood.

I am aware of the need to check the hose running to the ECU is clean and I will do that tomorrow. Is it recommended to disconnect that at both ends and blow it out?

I’m seeking other input as I start to turn my attention to this car from my XJ40 and X300 which have been pre-occupying me.

Separately I’d like some input on parts to fabricate testing fuel pressure although that does not seem to be an issue.


(Richard Dowling, 1979 XJ-S HE+5sp coupe, 1989 XJ-SC, 2003 XJ8 3.5L) #2
  1. Is it stalling because it is short of air ?
    Give it a touch of extra throttle to check that.

  2. If not, the answer might be lack of fuel.
    The surge tank in the boot can let enough fuel flow until the filter inside it gradually clogs. Then you stall. After a few minutes filter unclogs and you can start again - and run for a few minutes. A well known problem.
    Not sure a 92 car still has same fuel system.
    If it has you need check that surge tank and filter, lots of info about that in archives.

  3. Could be an ignition problem. I have never had a Marelli system but once again lots of info in the archives. Symptoms described don’t make it sound like a common ignition problem.

  4. The vacuum line to the ECU has to be working but not a common problem for them to fail. Blowing it clean will not detect a leak if it has one. Better to plug one end and suck to make sure it holds vacuum.

(Geekay) #3

I blocked off the end of the map sensor tube at the motor and the pipe holds vacuum. I made sure the pipe is tight at both ends.

I guess I could detach pipe at ECU and attach Mityvac and see if vacuum is present after warmup? Any issue with that as long as I don’t run motor long?

The odor is not fuel. It smells musty so it may be stale in the vents or rat leftovers.

I listened to the injectors and they are all pulsing although sound differs somewhat. I have put in fuel injector cleaner but as mentioned all works fine before warm up.

Could a faulty coolant temperature sensor or ECU be the cause?

This problem occurred prior to my spark plug change and vee clean out etc. so its not related.


(John) #4

Hi Geekay, where are you located?

The temp sensor is dirt cheap.

I like Richards suggestion about fuel starvation as the pump runs longer. If you live in USA, you can borrow a fuel pressure tester from AutoZone that has a brass T. You cut the fuel lines, yeah, but they can be replaced. I’d put it right in the cross over pipe hose.

My 92 doesn’t run at all so you’re one up on me.


(Geekay) #5

John - I ordered one this afternoon DBC3728. Less than $20. I’d like to have one hand as it also fits my XJ40. I also just located the Resistance Chart so I will test it tomorrow.20190228_215429_resized

I’d really like to build a tapping point so I can make a permanent fuel pressure test point.

After I’ve tested the CTS (and replaced if faulty) I’ll change the fuel filter.


(Steve) #6

The 1992 5.3L should have the same rail as my 6.0L. Here is an idea for you:


Takes a little fabrication, but looks good (:smile: ) and works well.

Good luck.

(Steve) #7

It is not common but possible. The two coils are in the see and if they are marginal, as the temperature builds up, misfires are happening more and more.

If I were the OP, I’d first establish that the fuel is delivered at the right pressure when the engine bogs down. Then, if this checks out, I’d focus my attention on the coils AND on the two ignition amplifiers on top of the radiator support. Some suggest putting ice-bags on those to check of the misfires disappear.

EDIT: Just saw the comment about the rat/mouse nest. #1 on the checklist would be to verify that you don’t have a wire that is chewed and touches ground

(Geekay) #8

Steve - thanks for the link. For some reason I cannot enlarge those images.


(Geekay) #9

Steve - after the original problem appeared and I spotted the arcing at the plug wire I decided to do the vee, change plugs etc. and check out all the wiring to the injectors. I checked as far back to the firewall as I could, replaced the connectors raised the loom with brackets hanging from the intake studs, and taped every inch of wiring with heat reisistant tape. I had to repair a couple of wires and the connectors going to the coils. I did replace the plug wires and the coils.

The problem appears to be the same now as before I did this work, although I could not get 2 miles away now from home as I did when the problem first appeared. So I’m ruling out coils etc.

I did not look at the amplifiers so I will have to read the archives on them.

Regarding the mouse - I trapped a mouse and then saw nuts at the rear of the vee. On inspection I cannot see any damage to wires - unless it was there before and is down the firewall.

I’ll check the CTS today and also vacuum lines and read up about the amplifiers. If the problem persists or goes away I will be changing the fuel filter. I will also look at fabricating for pressure test which I need to get done.

I did not see an issue with the rotor and cap but I’ll revisit that also if need be.

I also intend to have the injectors cleaned, but after this is sorted.

This car only has 54k miles on it so it’s not the std fix 'er upper. It needs to be on the road more.

Steve - I noticed in the images you posted in the album that you got 20 Hg in. of vacuum at the pressure regulator when warmed up. Can you or anyone else confirm that’s what I should be looking for on the 5.3". That’s something I can also test in the meantime.


(Steve) #10


There is a very long thread that is still currently active titled approx. “car still not running right after 2 months work…” Look it up. Extensive discussion of what manifold vacuum should be.

Or check this:

If the engine can not “breathe” properly, manifold vacuum is not where it should be

(Geekay) #11

An interesting read for sure. Today I will be validating the vacuum system overall and the MAP vacuum at the ECU and that the ECU is also holding vacuum. I will check resistance/change CTS. After that I will start checking the fuel side of things, beginning with the vacuum at the FPR.

For posterity I am attaching a image of what the ROM says about testing the Map/ECU. This is in the section referring to “Lotus ‘Epitest’”.



(Kirbert - author of the Book, former owner of an '83 XJ-S H.E.) #12

Hmmm. That process is intended to check for leaks in the manifold pressure sensor. It’s also supposed to check the peak vacuum on a decel, which is higher than normal idle vacuum. I’m a bit surprised that pass/fail is at 16, I would have expected it to be around 19-20. The stock V12 comes with overrun valves that pop open somewhere around 20-21, limiting manifold vacuum.

My '83 would usually read around 19" Hg at idle, but I had a manual transmission in it so there was zero load at idle. An automatic is driving a fluid pump at idle, so the vacuum will be slightly lower. I also had done away with the belt-driven fan which also reduces the load a bit, thereby giving a higher vacuum. I think I’d expect 17-18" Hg at idle in a bone-stock Jaguar V12 in good nick, and higher than that during a decel as described in this excerpt from the ROM.

(Geekay) #13

Well - my problem appears to be resolved although I’m in the situation where I did not get it pinpointed.

I first checked the ECU to make sure it would hold vacuum. It did. I trimmed the end of the hose and made sure it was tight into the ECU. I failed to test the car at this point. Aagh!

Then I removed the vacuum hose from the FPR. No fuel or debris present. I hooked up the vacuum gauge and got 16Hg at idle rising on decel. I did not record the exact readings. I trimmed the end of the hose and re-installed it.

I started the car. I waited for warm up. No problem. Dang.

I hooked up the vac gauge to the inlet manifold to screen capture pressure readings:

  • 16 Hg at idle - See image 1
  • Rising to mid 22-23 or so on deceleration from 2,500-3,000 revs. See image 2 - the reading was actually higher than the image but as I was screen capturing from video I couldn’t quite freeze it at the peak pressure.

I went on to change the fuel filter. After extensive warm up the engine sounds fine. Odd thing is that I could not detect that either of the vacuum lines was not solidly in place.

I will test drive it tomorrow afternoon.


P.S. After road testing to validate no issue I plan on sending the fuel injectors out for cleaning. At that time I will fabricate a fuel pressure test arrangement similar to what Steve has in his photo album.

(Geekay) #14

Well …or rather not well.

I went out this afternoon to start the car to go and get gas. After idling to warm it up it stalled. I notice a FF 13 MIL code which talks about Manifold Pressure. I was able to restart the car several times. It idles but the vacuum just drops all the way down when throttle is applied. I re-checked the ECU Map sensor and it holds vacuum.

I then noticed the gases coming from the exhausts are not even. The left one “puffs” in spurts and the right one is less pressure but even.

In searching for postings I ran into this thread which is akin to what I’m experiencing:

As a reminder I have changed
Plug Leads
New Fuel Injector Connectors & some wiring repair (minor).

The behavior I am experiencing is what seemed to be occurring before I did this work.

At that time I did not do all the vacuum testing etc. as I noticed some arcing at one of the leads and the vee clean/plug change was overdue. So at this point I’m not suspecting its related to any of the work I’ve done although I need to re-check everything clearly. As it was running well in the driveway yesterday after warmup it sounds like something is on the blink.

I need to digest the thread more closely but it sound as though I need to check coils and ignition modules. Any further advice is appreciated. I don’t want to flood a catalytic convertor for sure.

One other thing - this car has not been driven regularly enough. Could it be fuel/fuel injector issues?


(Steve) #15


Sorry to hear the smooth run was short-lived. If your A-bank is not running properly, then the poor vacuum will be expected. BTW, FF13 is indeed related to it – 36CU DTC FF13 PCMF MAP SENSOR SIGNAL – I am assuming the 26CU is the same in this regard.

Now, you need to check if this is ignition or fueling. Easiest test for ignition is to swap both coils. Test. If no change, swap the two ignition amps.

EDIT: By “no change” I mean the engine runs poorly, but the problem MOVES to the other bank.

Assuming you have good cap and rotor, I’d stop here and conclude that it is not the spark.

Testing the fueling will be more difficult unless you have a pressure gauge plumbed already and can verify that you have the right pressure.

Also, have you checked the exhaust back-pressure? Any chance the car has suffered partial Marelli meltdown in the past and the A-bank is not breathing properly, especially when the metal tubes heat up ?

(Geekay) #16

Steve - I replaced the coils and so I will check those after I check the amps. I am going to replace the rotor and cap. It makes sense for the next stage of the car’s life. I’ll also escalate figuring out best way to get fuel pressure checked.

I have not checked the exhaust back pressure.

The car has not suffered a Marelli meltdown. I’ve had the car since 42k miles and it ran well unitl 53k miles when the car first spluttered, which led to my vee cleanout etc.


(Steve) #17


The key-word I used in the earlier post was “partial”. If there was a misfire on the A-bank that was unnoticed for some time, the converters on that side will be compromised. With cars this old, there is a poor correlation, if any, between the chance for this happening and mileage.

Double and triple check everything - new parts (probably aftermarket does not always mean good parts)

To be certain, pull the crankshaft and the flywheel sensor and clean them too.

(Geekay) #18

Slight delay due to telephone company cutting my gas line (they’re rolling out fiber optic) :hot_face:

  1. I am revisiting everything in sequence. Re-read the Marelli section of the book as well.
  2. I changed the CPS in late 2017 (about 1k miles ago). It was faulty (low ohms). Flywheel sensor tested fine. I am going to re-test both tomorrow.
  3. Coolant Temp Sensor ohms verified.
  4. I switched ignition modules around. No impact on what is happening.
  5. I pulled the distributor cap and rotor. Photos attached. No doubt these are original. As stated I have a new Marelli rotor and a gasket on hand but did not purchase a cap.
  6. I believe its the B bank (drivers side US) that is not running good. It seems to sputter at the exhaust. The A is smooth. Both streams are hot. So it may be missing but the whole bank is not dead.
  7. After warm up the vacuum shoots to zero and the car would stall if I left revs at 2,500 or so. I’m not running long.

I have a video of when the car was running good the other day. So this seems like a loose connection or something that temporarily worked for one day. I’m looking everywhere but no luck yet.

I will have to switch the coils next I guess.

Regarding aftermarket parts - yes I agree. I have learned after many years to lean on reputable suppliers like SNG, MCL, CW etc. Although on the coils I bought SMP Intermotor.


(Geekay) #19

A couple of observations I would like input on:

  1. The 92 has both an ignition ECU & a Fuel (Injection) ECU. Both have vacuum lines to them:
    Ignition ECU
    MAPS Pipe to Fuel ECU
    Fuel ECU

a. Fuel ECU vacuum pipe is connected to the center of the cross pipe at the rear of the intake manifolds (in front of firewall).
b. I believe the narrow red pipe at the rear of the A bank is connected to the Ignition ECU.

I have tested vacuum does hold at both locations.

I want to test the wiring from the CTS to the Ignition ECU. That’s where I understand it connects according to the ROM supplement for 92 and later models. The pins on the ECU schematic are 6 (CTS Earth) & 19 (CTS). I intend to check continuity on the wiring and see if the ohm reading at the pins changes when I apply temperature to a good CTS (outside of the car in a non running mode).

The ROM does not show the pin connections for the fuel ECU. I am not at all clear as to what each vacuum is used for in startup and running mode. The ROM is a hair confusing.

Under the section on 6.0s it clearly says that a FF13 code is related to MAPS vacuum and is generated from the Injection ECU - “Looks for fluctuating MAPS vacuum signal and vacuum ‘v’ throttle position”.
I cannot see FF13 referenced elsewhere so my interpretation is that the 5.3 works the same way and that the FF13 code pointed directly to a vacuum problem detected at the Fuel ECU. Quote “Looks for fluctuating MAPS vacuum signal and vacuum ‘v’ throttle position.”

I understand the FF13 is the end result and the loss of vacuum can be caused by faulty CTS/Flywheel Sensor/wiring etc. rather than just vacuum leaks.

If I check pins 3 and 13 for the same OHM reading as the connector at the firewall (.680) that should validate the wiring from the connector to the ECU.

Likewise 1 & 2 for the CPS.

It would be nice to see pin connections on the fuel ECU too.



(Steve) #20

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