Member Since: 05 Nov 2015
Location: Leeds
Posts: 102
Manifold Absolute Pressure Reading - low?
Hi all,
I have a 4.4 v8 and wondering if someone could confirm what the Manifold Absolute Pressure reading at idle should be? I have searched on here and found the IID Tool suggested values state it should be around 100kPa, however mine is 40kPa at idle!
Context - the car has started showing the EML for P0171 & P0174 which is System Too Lean bank 1 & 2 respectively.
I have checked the vacuum lines and general air intake system whilst idling using carb cleaner to see if it causes the revs to raise, nothing. Visual check showed nothing either.
Obviously thoughts on the issue, but also the original question of what the manifold absolute pressure reading should be at idle.
Thanks in advance!
19th Aug 2020 10:59 pm
PROFSR G
Member Since: 06 Mar 2017
Location: Lost
Posts: 4657
It looks like you have a serious vacuum leak causing the mixture to lean out. I rarely see D3/4 V8's but I'm guessing that engine may have a PCV valve fitted to the intake manifold system. If you can confirm it is fitted I would investigate this as it is common on modern V8's and usually a simple enough fix.
Someone with better knowledge on that petrol V8 will soon confirm or otherwise.yµ (idµ - eAµ) ψ=mψ
19th Aug 2020 11:49 pm
Philsteward
Member Since: 05 Nov 2015
Location: Leeds
Posts: 102
Indeed it does have a PCV. I changed this in early 2016 when I rebuilt the heads due to VSR.
There is a different air noise in the engine bay, and I just found this thread which sounds rather similar...
I wouldn't have thought it would have failed already, but vehicle has done nearly 40k since the rebuild.
Anyone know how to test if it is okay??
20th Aug 2020 12:02 am
PROFSR G
Member Since: 06 Mar 2017
Location: Lost
Posts: 4657
The codes quoted in that thread regarding typical PCV failure are the same codes you have reported.
I wouldn't overthink it especially if you are certain all other manifold and intake joints are sound.
It's a common fault across many V8's not just LR. yµ (idµ - eAµ) ψ=mψ
20th Aug 2020 12:19 am
Philsteward
Member Since: 05 Nov 2015
Location: Leeds
Posts: 102
So an update to this, I have replaced the PCV for a genuine new one, and I have replaced the MAF with a genuine as well.
Seems to have made no difference, still getting the system lean fault codes and the Manifold Absolute Pressure is still 40kPa, should it really be 100kPa, is there anyone with a v8 and IIDTool who could check for me?
Keen to get this resolved for obvious reasons!
27th Aug 2020 4:22 pm
PROFSR G
Member Since: 06 Mar 2017
Location: Lost
Posts: 4657
40 kPa is still way too low and yes it should ideally be at least 100 kPa at idle.
100 kPa is about 1 Bar, not too far from the atmospheric pressure right now, and what you'd expect to see if the engine was switched off!!! You're current reading of 40 kPa at idle is way off the mark.
Try disconnecting the MAP sensor! This hopefully will cause the PCM to go to a pre-programmed default setting for MAP, and if so you might see an improvement. If you do, there's a good chance the MAP sensor is either blocked with crud or it's kaput. If blocked don't push anything into it to clean, just use brake or carb cleaner. yµ (idµ - eAµ) ψ=mψ
27th Aug 2020 7:41 pm
Philsteward
Member Since: 05 Nov 2015
Location: Leeds
Posts: 102
Great next step suggestion thanks. I did notice the barometric pressure reading was 101kPa, does that come through another sensor? It didn't alter with engine revs, which makes sense I think.
I only managed to find info on the MAF for the diesel engine on this site, but found the below link which is very helpful, although talks a lot about hell!!
Will report back once I have less skin on my knuckles and looked at the sensor!!
28th Aug 2020 11:08 am
PROFSR G
Member Since: 06 Mar 2017
Location: Lost
Posts: 4657
Yeah check the sensor first before replacing it. On diesel's they get blocked much more easily because of the crud build up with the EGR gasses being routed through the "oil wet" inlet manifolds.
Yours may not be blocked, but it could still be faulty. Either way, the sensor should display a scale of operating value with the engine running at different rpm. If not, either it or the wiring to it it has a problem. One caveat though, there should be a dtc stored to suggest a problem with it!!
So it might be worthwhile considering a fuel pressure test before you start throwing more parts at it. A good diagnostics tool and testing is your friend!
You just have to your way through it logically and systematically, and it will yield a result. ....Eventually yµ (idµ - eAµ) ψ=mψ
28th Aug 2020 1:40 pm
Philsteward
Member Since: 05 Nov 2015
Location: Leeds
Posts: 102
Thanks again. Will do as you suggested.
When I rev the engine up to 3k rpm the sensor reading drops from 40kPa to 25kPa, with a steady drop as the revs increase from idle. Is that right? If it were on 100kPa, does it drop slightly when revved?
Understand re fuel pressure, however as the car is LPG and the issue presents on both fuels, I figure it must be a sensor. As it is on both banks, I have ruled out the O2 sensors. Could be wrong of course!
I have no DTC for the MAP. Surprised as if it should be 100kPa at idle and it is 40kPa surely that would show an issue?
28th Aug 2020 3:55 pm
PROFSR G
Member Since: 06 Mar 2017
Location: Lost
Posts: 4657
LPG?
OK I was not aware of that, I don't know if it is relevant or not but it might be. ?yµ (idµ - eAµ) ψ=mψ
28th Aug 2020 4:58 pm
Philsteward
Member Since: 05 Nov 2015
Location: Leeds
Posts: 102
Sorry, thought I had it on the original post. The issue appears on both fuels. I did get a very minor hesitation on idle when warm, and a bit of a hesitation when accelerating with the caravan attached. This happened on LPG, but LPG tends to present fuel mix issues before petrol.
This stopped when I changed the PCV, but the fault for lean mixture still comes back.
The LPG is nothing to do with the issue as it still presents on petrol.
28th Aug 2020 5:07 pm
PROFSR G
Member Since: 06 Mar 2017
Location: Lost
Posts: 4657
I'm just wondering if there is a problem with the hardware side of the LPG system that might be causing your issue?
In the meantime check the MAP sensor as per my earlier post and see if there is any change!yµ (idµ - eAµ) ψ=mψ
28th Aug 2020 7:21 pm
Philsteward
Member Since: 05 Nov 2015
Location: Leeds
Posts: 102
I've checked the LPG injector pipes (if failed could cause an air leak) and I have checked the one vacuum pipe for the same reason. That's it for hardware which interacts with the engine.
Engine had a rebuild in early 2016 and I replaced all the LPG pipes then, and most other vacuum pipes and coolant pipes etc, so effectively all less than 5 years old. Doesn't mean they haven't failed but unlikely I'd like to think.
Will check as you say and report back. Thanks for the help.
28th Aug 2020 8:04 pm
PROFSR G
Member Since: 06 Mar 2017
Location: Lost
Posts: 4657
yµ (idµ - eAµ) ψ=mψ
28th Aug 2020 8:05 pm
Philsteward
Member Since: 05 Nov 2015
Location: Leeds
Posts: 102
So to offer you an update should anyone have similar issues...
I did get to the MAP sensor, the "gates of hell" comments from the post from another site did keep me chuckling whilst trying to get to it, what a pain. Looked absolutely fine, bit of oil, and the car ran like an absolute pig when the sensor was unplugged, so I assume working to some degree.
Taken it for an MOT now as that is due in a month and it failed on high CO2. I also noticed on the way to the MOT station that the long term fuel trim when driving was getting high and imbalanced, 11% on one side, 14% on the other. And when idling it shot up to 22%.
Got a smoke test done, and the smoke was pouring out of the intake baffle, another vacuum connector on top of the throttle body/near the EGR, and generally from round the driver's side of the inlet manifold.
Anyone had any experience of this? I think I'll get a new baffle, double check the vacuum connector which is struggling but I'll have to remove the plastic intake manifold and see what's going on.
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