Are There Any Engine Diagnostics Techs Among Us?

[Porschemaniac]

Hi there,

I wonder if someone may be able to share some wisdom on this?

My lovely old Jag (1995 4.0l X300 Sovereign) recently struggled to pass an MOT - on emissions. The tester (as luck would have it, a fellow Jaguar enthusiast) spent several minutes revving the engine to get a pass.

The test showed that the mixture is rich, however, oddly, it idles a little fast - suggesting maybe an induction air leak.

After connecting my ELM 32 scanner, I am getting some readings, which I think may throw some light on this.

The MAF is reading 0.01 lbs/min at 2016 rpm. In fact, it does not seem to go much above this - even up to about 2200 rpm. Now, I don't know much about diagnostics - but that seems very low to me. In this same frame of data, the bank 1 O2 sensor is reading 0.045V and Bank 2 sensor 0.040V. Two frames of data later (with RPM at 2048 rpm), O2 sensors are reading 1.245V on Bank 1 and 1.205V on Bank 2. Engine Coolant Temp is 84 degrees C.

Additionally, Long Term Fuel Trim (Bank 1 and 2) are both consistently at 40.6 % - which, to me, seems high.

This leads me to suspect that the MAF is either faulty - or just getting a bit tired. I guess that this Engine Management System uses the 'hot wire' type MAF? I have heard of these getting choked with soot and loosing their efficiency - and wonder if this may be what has happened.

Assuming that the MAF is reading incorrectly, I presume tha this could lead to the EMS adding more fuel.

I do know that my engine has usual cracked exhaust manifolds (a very common problem on these cars) - and, again, I wonder if this complicates things (misleading the Oxygen Sensors as not all of the escaping combustion gasses are passing by them - some making their way out through the cracked manifold).

As may be obvious, I am not very experienced at interpreting OBD data, so any wisdom will be greatly received.

Thanks,

Ian.

 

[BChild]

I do know that my engine has usual cracked exhaust manifolds (a very common problem on these cars) - and, again, I wonder if this complicates things (misleading the Oxygen Sensors as not all of the escaping combustion gasses are passing by them - some making their way out through the cracked manifold).

More likely exhaust gases out as well as air in and as a result the lambda control is trying to restore the fueling to stometric and having fun with it!!

 

[Porschemaniac]

Thanks BChild - I did wonder if that was the case.

Certainly makes sense.

Best I get those manifolds welded ASAP!

 

[Bigjoe]

Hi Ian,
The cracked manifolds may well affect the o2 (lambda) readings due to some escaping exhause gases,to what extent depends on the cracking.

Have the lambda sensors been checked themselves?

It does sound as if the MAF could well be faulty,as its a hot wire system have you tried carefully cleaning the wire etc with a soft paintbrush (engine off) to see if the readings change at all?,likewise check all the hoses and wiring connections.

As for you obd readings,i have'nt done anything with a jag for many years,have you tried the jag owners club,do they not have a technical line?

Sorry i cant be of more help on this one,jags are lovely to look at but i always hated working on them

Joe

 

[Porschemaniac]

Cheers Joe,

Yes - I make you right about the inversely-proportional relationship between how nice these cars are to look at, versus how frustrating they can be to work on.

I must admit that I have not checked the Lmabda Sensors. How do you test these? Hopefully, it's as simple as a resistance measurement - although, in view of what I said in the previous paragraph, I suspect not!

Will certainly try cleaning the MAF. To be honest, I wasn't sure if you could try shooting some carb cleaner down it - or whether that would end up doing more har, than good.

Thanks for the reply,

Ian.

 

[Howburn]

I would go with the cracked manifolds, result O2 sensors measuring a lean burn.

Try to get some readings at the sensors I normally do this with a scope, but you could try with a multi meter.

You could to some extent test the O2 sensors on the car, when you have your diagnostic machine plugged in, try pumping the brake pedal, should force the readings lean, see if the fuel trim responds, or alternatively force it rich with a small propane torch.

Can you get some exhaust paste to fill the cracks?

I wouldn’t be keen cleaning the MAF with a paint brush safer in my opinion with carb cleaner and compressed air, but my bet is the manifold.

 

[Bigjoe]

A blast of carb cleaner won't do any harm,ive cleaned contaminated hot wires with a brush before, i would second (or third) the manifold cracking as a likely source.

 

[Porschemaniac]

Cheers Howburn,

Yes, the manifolds certainly seem to be the odds-on favourite.

I'm using PCM Scan, which provides graphical output. Don't know if this gives any insight into how the Lambda sensors are performing? I guess it emulates the function of a scope.

If not, I'll have to break out the old Gould Scope I've had lurking at the back of the garage for a few years now.

Thanks again,

Ian.

 

[Porschemaniac]

Thanks Joe,

Off to Halfrauds tomorrow for a big can or carb cleaner and some exhaust putty (just to prove the theory before I invest in new manifolds - or whip these off and weld them up).

Cheers,

Ian.

 

[tigler]

I must admit that I have not checked the Lmabda Sensors. How do you test these? Hopefully, it's as simple as a resistance measurement - although, in view of what I said in the previous paragraph, I suspect not!

Once at working temperature , about 600 deg C , the sensor will produce a DC voltage signal of about 0.9 Volts on a rich mixture which will fall abruptly to about 0.05 volts on a weak mixture. It switches very rapidly at working temp , it is virtually a square wave on a scope. The voltage can be measured ok on a multimeter. Lambda sensors are a service item really , they degrade over time and although the ECU can compensate to some extent the engine tune suffers eventually.

 

[Porschemaniac]

Cheers Tigler,

Looking at my diagnostics software, it appears (to me at least) as if this is what is happening.



Maybe, in addition to welding the manifolds, I should think about replacing the sensors?

Cheers,

Ian.

 

[Howburn]

Just had some more thoughts, the output of your pre and post cat sensors are very similar, the pre cat sensor should be switching between .1 and .9v and providing the cat is up to temperature the post cat sensor should be flat lining at around .45v I think, showing the cat is working efficiently. The post cat sensor should only fluctuate when you introduce a lot more fuel or air, it really is only there to monitor the cat, not control fuel trim etc.

Since your post cat sensor is sort of mirroring the pre cat sensor this could indicate that the cat is faulty or on its way out, there may be nothing wrong with the sensors and they are actually giving you a correct signal that something is not quite right, but as yet have not triggered the MIL light. And the garage emissions machine was correctly picking up a fault.

Having said all that, plug the manifold leaks first, don’t change the sensors yet.

 

[bill_gj]

I don't think this system has pre- and post catalyst sensors - this wasn't necessary in 1995. It's just O2/HEGO sensors in the manifolds.

I can't say that I share the theories about manifold cracks leading to sensors reading lean. Conditions in the exhaust manifold will lead to gas trying to get out rather than let air in.

As TIGLER said - the only way to really check the fnction of the O2/HEGO sensors is to look at their output with a scope. A multimeter/DVM will not do anything other than give an idea as to whether they're working properly or not. A steady output of 1.2 volts says that either the sensor is u/s or the measuring equipment is not adequate.