Car still idles rough/runs rich after 2 months work - I'm at a wall

(Michael Garcia) #244

removing the segments between the manifold and the cats would be the easiest tell tale…
most of the accumulation of carbon
will be in the mufflers near the rear tires though…

treat the nuts a day before
take about two hours time
to remove them

start it up
let it achieve normal running temp
see what it does…
probably light it up outside though
it will shake the foundation otherwise

not that there is anything wrong with that, but if you have pets, you might want to have your wife take them and the kids to moms for a while…

(Kirbert - author of the Book, former owner of an '83 XJ-S H.E.) #245

Oh, it can be that bad! A Marelli meltdown can easily turn the core of a cat into a molten glob that all but totally plugs up the exhaust system. And another reported failure, believe it or not, is that the inner lining of the straight pipe from the cat rearward can collapse internally and all but totally plug up the exhaust system.

The easy and quick test is to unscrew the oxygen sensors and start 'er up. It’ll sound like a WWII fighter plane, but it should be immediately apparent that it’s running much better.

In the particular case being discussed in this thread, we’ve already confirmed that changing the vacuum and/or CTS inputs does nothing. I don’t believe that indicates a plugged exhaust system; it indicates an ECU that is shot.

(Michael Garcia) #246

could be both
sometimes one thing
perpetuates another…

(Michael Garcia) #247

may be
a loose ground
or grounds…

(Greg) #248

Not much of an update, now out of nowhere, car won’t start. (only thing I did was adjust throttle plates…one was more than 0.002)

Coil and plug sparks excellent, nice and blue.

Compression good (only tested front six cylinders)

Fuel pressure good.

I can hear injectors clicking while cranking.

Haven’t touched timing in weeks, but last checked it was good.

It’s always been a bit hard to start, IF ECU is opening injectors at full pulse all the time, and if it’s doing that while trying to start, would that flood it easily and make for a difficult start?

Am going to send off ECU to AJ6. Will follow up once I hear back.


Conclusion: Less air at idle is just too little to support even poor running. Reverse your throttle adjustment to see if this is actually a causal relationship. What happens if you turn it over while holding the throttles at a steady half open?

Yes. If you’re getting too much fuel at startup ( as well as other times ) of course it would cause a very difficult to burn mixture.

All things summed up, you’re wise to have the ecu tested. It’s certainly a possibility it’s failed. Eliminate that possibility.

(Greg) #250

Yeah, it was cold too, only 38F, so that’s probably not helping matters. Last weekend it was 50F.

After trying to start it about 3 times, I pulled a plug, it was wet with gas. Yeah, I tried holding throttle wide open, and partially open, no luck. It must be flooding almost instantly.

ECU is sent out.

(Richard Dowling, 1979 XJ-S HE+5sp coupe, 1989 XJ-SC, 2003 XJ8 3.5L) #251

Finished rewiring the coupe last weekend.
In theory nothing should have changed, but it did.
Spent 10 minutes trying to start it. Would fire on a few cylinders when cranking, but then die after one or two revs and still only a few cylinders firing.
I had plugged my laptop into the Motec EFI system, and it has a diagnostic feature which lets me exercise the ignition system and injectors ( fuel pump off ), both O.K.
So check list initiated.
Ignition and injectors wired correctly and working.
Fuel pressure good, have pressure gauge in engine bay.
Smell exhaust, very little fuel smell.

Conclusion: Injectors are checked O.K., but when the Motec is trying to run the engine under normal program seems not to be injecting much fuel. I ran out of time to look further.

Later took a look at my wiring diagram and suspect have a wrong connection for the coolant sensor.
Probably caused Motec to get a default value saying engine at 100deg C. That will cause weak mixture. Will check it out at the weekend. I also found that I can change the timing with the laptop from -50deg thru to +50deg. If I get the engine running will see what that does.

(Philip Lochner) #252

PC connected to Motec should display the CLT value it “sees”. If that value does not seem realistic you KNOW there are issues.

If you are sparking through a dissy, (EDIT) where the actual spark event is controlled by an ECU (typiclally using a 36-1 etc trigger wheel) (stop edit), the number of degrees you can advance or retard is LIMITED by:

  1. the WIDTH of the rotor tip;
  2. The WIDTH of the HT terminal in the dissy cap;
  3. The rotor phasing: ie the physical position of the rotor tip relative to the HT terminal when the spark occurs.
  4. If your dissy’s mechanical advance is locked out, the range of spark /advance is FURTHER LIMITED and even more affected by rotor phasing - which is why locking out mechanical / centrifugal advance is NOT a good idea.

If you spark through coil packs, any amount of advance/retard is possible.

(Dick Maury) #253

Actually this is not totally correct as to the total advance being limited by the rotor tip width. The firing timing is controlled by the springs and weights on the earlier cars including the XJS Pre-Marelli. As the advance occurs, the springs and weights cause the timing to advance relative to the crank. The spark still occurs in time with the rotor and cap as the reluctor is phazed with the rotor. The total change happens relative to the crank. On the Marelli cars, there are no advance weights so the limit is in relation to the rotor tip width. As far as the coolant temp sensor, simply disconnect and put a jumper on the plug. This will simulate a fully warmed up car. An open circut will make the ECU think the car is in the Artic and run very rich.

(Philip Lochner) #254

Dick, you have a point, but my statement was “totally correct” because my reply was in response to Richard’s post where we specifically focus on the situation where the actual spark event is NOT CONTROLLED by the dissy but by an ECU and merely being directed to the correct spark plug by the dissy.

Noting your point, I will edit my post to make it clear under what circumstances my statement holds true.

(Richard Dowling, 1979 XJ-S HE+5sp coupe, 1989 XJ-SC, 2003 XJ8 3.5L) #255

Quite correct Philip.
Last weekend I had very little time to pursue why the coupe V12 would not fire up.
This weekend after the usual minor struggle to get the old Windows 7 laptop to deal with the even older Dos based Motec there was lift off.
Straight away I could see the MAP sensor was reading 23ins and the engine coolant was 80deg C while the engine still dead.

I found a ground wire not connected and also 2 crimped contacts in a couple of plugs not pushed far enough to fully engage. It is really handy to have an EFI system where you can read all the variables before cranking. I guess any modern aftermarket system will allow for this.

Two or three seconds of cranking and the engine fired up. Coolant and air temps around 20deg C, and at 600 to 650RPM the engine pulls 15ins vacuum.
Just for kicks I moved all the plug wires around 60deg lagging.
As Philip mentioned, to do it using the Motec system not wise since I can only move it 50deg lag and that may not guarantee the rotor tip lines up with a post in my dizzy which is locked, no mechanical advance. At 60deg it would line up with the next post, it has to.

In any event, 3 attempts at cranking did not get the engine to run. Still pretty confident when I had a problem on first starting up the V12 HE engine in the 1979 coupe, many years ago, I had all the plug wires 60deg lagging. It ran but very rich and without the usual V12 exhaust note.

(Greg) #256

It was the ECU!
It is now in the talented hands of Roger Bywater. He has only taken a quick look so far:

A quick check shows that it is indeed not responding to the vacuum signal but the pressure sensor seems to be OK.
Other functions (air and coolant temp, throttle pot, etc) are also inoperative so it might be an ADC fault.

(phillip keeter) #257

Wow! What a surprise.

(Paul M. Novak) #258

This is encouraging news, that you may have found the source of your engine problems. I hope that your car runs properly once you get a good ECU installed.


(Robin O'Connor) #259

Get ready to burn rubber :slight_smile:

(Paul Wigton) #260

Not to mention, all the carbon built up in the engine and exhaust… but, dat dare’s FUUUUUN!


(Greg) #261

Ha ha, I wish. Brakes and cooling system still need attention, and the tires are too old for the highway right now.

I hope the ECU does the trick. But I learned so much about this engine along the way, so it was worth it.

(Michael Garcia) #262

l am happy for you Greg,
maybe Bywater
can tune the ecu where it passes emissions, yet customizes it to your driving style…
it is an afforded luxury
as opposed to a necessity,
in other words
you have another car,
you may choose
to drive it daily
but it is not
your daily driver.

l have always wanted
to get my ecu tuned…

(Greg) #263

No more emissions here in WA if the car is over 25 years old! May ask what the price difference would be to fix it vs. his super enhanced version.