P38A Pressing the accelerator slows the engine

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Bertie Wooster

Member
Posts
74
Location
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Hello,
The engine on my P38 (petrol, 4.0 litre, Bosh) is not running correctly. Normal pressing of the accelerator usually slows the car. By pressing very slowly, and gently I can increase speed. Flooring the pedal also increases speed.
Hills, even gentle inclines, are a struggle.

Error codes are (after clearing stored errors and a short drive):
PO 134 - Lambda sensor upstream catalyst bank 1 drive cycle C, signal missing
P 1000 - Lambda sensor upstream catalyst bank 2 drive cycle C, signal missing.

Is it time to replace the oxygen sensors or, because a signal from both banks is missing, could it be something else?
Many thanks,
Bertie
 
Definitely sounds like it is running lean. Ignoring the codes for a minute, how are the fuel filter and the fuel pump in tank screen? What pressure do you have at the fuel rail? It should be about 40 PSI. A failing or partially blocked pump will give similar symptoms, which may lead to the errors you see.

If the above is all in order, then the error code would point to one of two things in my book. Either the O2 sensors are toast, they are a consumable part, or you have dodgy connections somewhere. They both have plugs clipped to the side of the sump. You could start there and give them a clean and then see what codes you get. The likelihood of both having dodgy connections at the same time though isn’t high.

If you decide to replace them, make sure to re-set the adaptive values when you finish.
 
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Agreed, upstream being the engines emissions, check the wiring for both. If you can also check with diagnostic equipment, check for a rise and fall pattern from each sensor. If no pattern confirm with an outside fuel input I. E from à blowtorch to see if the pattern changes. If no difference with an additional fuel source the sensors could well be pooped.
Take a look at your fuel trims as well, these will drop dramatically when the extra fuel source is added and will try to accommodate the increased emissions from the engine.
Your sensors could be hilighted due to a lack of emissions as our mate here says and not necessarily duff sensors.;)
 
Hello Marshall8hp and Mark, many thanks for your suggestions. I will make a start with checking fuel supply and pressures.

In the mean time I cleaned the MAF sensor yesterday and the engine now runs very much better and the error codes have changed. I now have P0172 and P0175. I believe that these suggest the mixture is too rich. I think i will buy a new MAF sensor and fit this before trying further investigations if you think this makes sense.

Thanks agoan,

Bertie
 
Hi Mark, the air filter looks fine (unfortunately - that would be an easy fix). :)
David, I've just reset the adaptive values (after cleaning the MAF sensor) and am getting P0134 and P1000 faults - both suggesting no input the oxygen sensors. I'm very new to code reading and working out what they mean but research seems to suggest that after re-setting adaptive values a good run (possibly days of driving) is needed before the ECU sorts itself out. I've ordered a new MAF sensor and will fit that when it arrives, re-set the adaptive values again, and drive for a few days to see what happens. Is this sense?

If these faults persist I'll check the wiring and connectors and if these look OK I guess I'll have to buy two new sensors. Are all sensors equal? Prices seem to vary a lot from around £42 from John Craddock to genuine ones for £211 from Rimmer Bros.

Thanks again for all the help.
Bertie
 
Hi Mark, I haven't tried the adding fuel trick yet, and I'm going to have to ask if you'd mind explaining this in a bit more detail. How do I add the fuel? And where do I add it? Also, what do I look at on the Nanocdom when this is added, the live oxygen sensor readings? Is this something I can do on my own or will the fuel have to be added when someone else is looking at the readings?
Also, the fuel trims. I guess I'd look at the live fuel trim data when fuel is added.
Cheers,
Bertie
 
OK.... Deep breath!!
Your fuel trims are in charge of the emissions the cat feeds on. A certain amount of oxygen is allowed into the cat to help it burn the gases. This is why the lambda sensor has a pattern that goes up and down.it reflects a lean and rich condition.
Your fuel trims will always try to hit the ideal fueling. When you add a separate or external fuel via the inlet after the maf sensor with the engine running the fuel trims will detect extra emissions via the lambda sensor and the fuel trims will reduce the amount of petrol in the mix. If the fuel trims don't react to the extra fuel in the mix the lambda sensors pre cat are defective. Unfortunately on the nanocom I'm only familiar with the diesels... Sorry, on the autlel mx808 I can read this on various other cars but never had a v8 in the shop other than the 4.2sc. So I can't advise any further than that.... :oops:
 
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Bertie, further to Mark’s comment. If you go to the ECU Fueling page you will see live data for the O2 sensors. It should show them flick about between 0V and 5V, a voltage which goes back to the ECU to tell it if the engine is rich or lean, so on the next injection cycle it makes a tiny adjustment to the quantity of fuel injected.

Question, when you look at the live data, do you see the O2 sensor voltage flicking about? From your description, if the codes are correct, they would both be constantly on 0V.

As to your question on how to add the additional fuel, some form of hydrocarbon introduced into the air intake. So squirt a little bit of carb cleaner into the air intake when running, or a little bit of butane from your BBQ gas bottle. It will lead to a rich mixture, and if your O2 sensors are working, would would see that reflected in the live data.
 
Mark, excellent description, many thanks, that's going to be printed and stuck on the wall.
David, even more good info, thanks very much.

Results of experiments:
Oxygen sensors
At 500rmp flicked between 0.4 and 0.5 and occasionally spiked to 0.7.
Gentle revving gave reading of between 0.3 and 0.7
Steady 2000rmp gave readings between 0.25 and 0.35

I then disconnected the air intake between the MAF sensor and inlet manifold to inject butane after the sensor but the engine died and stalled. So reconnected and disconnected the filter from the MAF sensor.
without butane flicked between 0.25 to 0.7
with butane (continuous supply) more static between 0.7 and 0.8

Trim
at 500 rpm hovered between 0.95 and 1.1 with air filter disconnected
with butane (continuous supply) readings fell to between 0.75 and 0.8
removed butane, engine almost stalled then picked up to give readings around 1.2
reading stayed at aroundn 1.2 for four monutes then fell to 0.99

So, what do you think? Is this all this to be expected? Very many thanks,

Bertie

PS If new oxygen sensors are needed then thanks Grrrrrr, I'll avoid the cheap ones. :)
 
We could do with another 4.0ltr and some fuel trim Info, ate there any air leaks jn the intake or any little vacuum pipes split. The fuel trims should be trying to bring the numbers down as much as possible. If there was an air leak the trims would lift to try and balance the mixture. Also a lack of fuel as David suggested with a dirty or blocked fuel screen. Did you check the fuel rail pressure as well?
 
Update - I fitted a new MAF sensor yesterday and after a couple of short drives (5 miles and 10 miles) no errors have been logged. The engine is also running better than it has for a long time. I'm hoping that this has solved the issue - do you think I'm right to be optimistic? I don't know why a faulty MAF sensor should appear as oxygen sensor faults.

Mark, I had not checked the fuel pressure, I don't have a pressure gauge. Perhaps my next purchase :) I can't find any air leaks or split/damaged pipes so hopefully no error massages suggests that's all's well.

Very many thanks for all your helpful replies, I've learnt a lot from your generous info. As an aside, does anyone know of a good book which explains ECUs, error codes etc and how to try and interpret them? If I could understand what's going on a little better then I may feel a little less clueless when staring at the Nanocom.

Cheersw,

Bertie
 
I don't know of any specific book however I picked up a used copy of a textbook that was used at an automobile mechanics trade school and it discusses these sorts of things. You might be able to find one on Amazon as they sell used books through third parties. Buy the most recent one you can find. May set you on a new career.
 
Sounds interesting, any idea of the title? A new carer is what I want, but when talking to mechanics a fraction of my age I feel as though I've been left a long way behind. Carburettors and 1960s tractors (with no electronics) are more my thing. :)
 
The error codes for the lambda sensors in this case was a symptom of something upstream. The error codes don't always mean the actual thing listed is the actual cause and by testing the lambda sensors as you did helped to prove they were doing their job. With the maf fitted now, take a reading and get familiar with its behavior on the live data. That'll give you a good understanding of what should be like for future reference.
I guess the evidence started to show when you had cleaned the maf and gained some power. Nice one:D
 
Good idea Mark, I'll look at all the live data and make a note of what's happening. Many thanks for all your help, and also many thanks to all who gave me their ideas - I know a lot more about eror codes and live data than a week ore tow ago. :)
Cheers,
Bertie
 
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