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Old 11-24-2020, 09:46 AM   #1
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Confused about MAF and LTFT readings

Hi all,

Thanks to this great forum, I've learned a lot about our workhorse W22 chassis, and I also decided to attach my Bluetooth OBD adapter so I could watch my fuel trims while driving. Our 8.1 has been converted to run on LPG for some years now, as it makes driving much cheaper, and is a popular mod!

When travelling back from Spain a couple of weeks ago, I saw that my MAF reading was only showing about 160g/s on WOT - though it would gradually climb higher if I kept the throttle open. But my STFT and LTFT were pretty normal - this was all while running on LPG.

I was obviously concerned by the MAF reading, so I tried not to use too much throttle on the way home. Once here, we've changed the old air filter and cleaned the MAF, which appears to be an original Delphi.

We took it out for a test drive earlier, and I kept the engine in petrol-only mode. Once the engine is warm, the idle reading is 8-11g/s, and when I am able to pin the throttle the reading starts around 150g/s and gradually climbs to just over 200g/s - that takes around 10-15s or so to reach. I don't have any hills nearby where I can hold the throttle open for a longer period of time though. During the entire test drive, the LTFT trims went crazy though, up to 22 on both banks at one point.

I was intending to order a new MAF, but if it's getting over 200g/s after a while does that indicate it's working properly? And if it is, why might I be getting such figures on the LTFT?

Thanks for reading my slightly rambling question!
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Old 11-24-2020, 08:35 PM   #2
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If your front oxygen sensors get up close to 900mv on wot you may have the beginning of clogged catalytic converters. To test for a pluged exhaust remove a front oxygen sensor and put in a vacuum/pressure guage. Anything over 1.5 psi indicates a clogged exhaust, most likely the catalytic converter.
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Old 11-24-2020, 09:35 PM   #3
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Start with a fresh MAF, it shouldnít struggle to reach 200 g/s. You can also use a baseline of what it reaches at the 1>2 gear shift at WOT. It should reach at least 230 g/s. Fuel trims at WOT arenít relevant as you go into open loop and the computer ignores the oxygen sensors.
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Old 11-24-2020, 10:51 PM   #4
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Has anybody in the United States converted their 8.1 to burn LPG?

I had never heard of doing that before I read Greedo's post.
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Old 11-25-2020, 12:29 AM   #5
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Quote:
Originally Posted by Arch Hoagland View Post
Has anybody in the United States converted their 8.1 to burn LPG?

I had never heard of doing that before I read Greedo's post.


I havenít. I know during the fuel crisis of the 70ís that was a pretty simple conversion on a carbureted engine. My uncle did it to his pickup. Essentially turned it into a dual fuel. Could run on gasoline or propane. I can see where it might be more economical where the price of gasoline is higher than propane (lpg) for the same energy density. But with gas at 1.67 a gallon I canít see the justification.
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Old 11-25-2020, 12:47 AM   #6
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Greedo...I have a 75 gallon gasoline tank in my motorhome.

Where do you store your LPG and how much can you store?
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Old 11-25-2020, 06:44 AM   #7
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Quote:
Originally Posted by Arch Hoagland View Post
Has anybody in the United States converted their 8.1 to burn LPG?

I had never heard of doing that before I read Greedo's post.
Quote:
Originally Posted by Kluffinator View Post
I haven’t. I know during the fuel crisis of the 70’s that was a pretty simple conversion on a carbureted engine. My uncle did it to his pickup. Essentially turned it into a dual fuel. Could run on gasoline or propane. I can see where it might be more economical where the price of gasoline is higher than propane (lpg) for the same energy density. But with gas at 1.67 a gallon I can’t see the justification.
It's very common on RVs and US imports to the UK due to fuel prices.

As of today, regular petrol is $5.91 per US gallon. Whereas LPG is $2.42 per US gallon. It costs about $5,500 to convert, and can then run on petrol or LPG as necessary. There's a new switch with a built-in fuel gauge on the dashboard. It's fully electronic and uses a piggyback ECU that adapts the instructions for the LPG injectors.

Due to the rise of EVs and also the push to use diesel in Europe over the last two decades, LPG conversions, and therefore filling stations are getting rarer. Our next RV will almost definitely be a diesel, as diesel is almost at price parity with petrol in the UK, and cheaper in Europe. The lowest we've seen is $4.31 per US gallon while we were in Spain.

Quote:
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Greedo...I have a 75 gallon gasoline tank in my motorhome.

Where do you store your LPG and how much can you store?
We still have the original petrol tank. Most people put an additional tank behind or near the existing petrol tank. However, the installers on our RV had it put behind the driver's side front wheel - replacing two small lockers. It's got a total capacity of 58 US gallons, but as you only ever fill to about 80%, that's a max usable capacity of 47.5 US gallons. Enough for between 300 and 360 miles depending on the type of road we're on. We use the same pumps to fill the engine and domestic tanks, which have an automatic cut-off at the correct fill level.

Here's the additional injectors and final stage fuel filter under the dog house:


Here are the twin vaporisers to convert the LPG to gas for the injectors. They are plumbed into the cooling system as they use heat to do this. Consequently, the engine always starts on petrol and switches over to LPG when the coolant reaches about 113F.
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Old 11-25-2020, 06:53 AM   #8
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Originally Posted by Porkchopfat View Post
If your front oxygen sensors get up close to 900mv on wot you may have the beginning of clogged catalytic converters. To test for a pluged exhaust remove a front oxygen sensor and put in a vacuum/pressure guage. Anything over 1.5 psi indicates a clogged exhaust, most likely the catalytic converter.
I do have an O2 sensor readout on my data screen, but I wasn't paying attention to that when I went for my test drive. I will make a new dashboard so that I can see those levels at the same time.

Quote:
Originally Posted by Ultra RV Products View Post
Start with a fresh MAF, it shouldnít struggle to reach 200 g/s. You can also use a baseline of what it reaches at the 1>2 gear shift at WOT. It should reach at least 230 g/s. Fuel trims at WOT arenít relevant as you go into open loop and the computer ignores the oxygen sensors.
The LTFT was reading in the high teens and low twenties even under part throttle conditions. Though not when maintaining speed on level ground, when I'm just holding speed, the LTFTs dropped to between 0 and 5.

I need to come up with a test plan and route to do next time I'm able to take the RV out. I have seen some people recommend trying to maintain particular rpm for 30 seconds or so before taking the reading - would that need to be on an uphill stretch so that I get a loaded reading?
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Old 11-25-2020, 01:57 PM   #9
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Interesting for sure. The times they are a changing as Bob Dylan sang.

Have you been on the Autobahn and how fast did you go?

Oh, by the way, your two links to the pictures don't work.
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Old 11-25-2020, 03:03 PM   #10
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Quote:
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Interesting for sure. The times they are a changing as Bob Dylan sang.

Have you been on the Autobahn and how fast did you go?

Oh, by the way, your two links to the pictures don't work.
That's strange about the pictures, they should appear as the link is public.

I haven't taken the RV to Germany yet, but I'll probably stick to 60 mph or so, 56 is the truck limit across most of Europe, it's pretty economical to drive at that speed, and I am in control of the overtaking! Car wise, last time I was there the limiter cut in around 160 mph or thereabouts, however, I find it more comfortable around 90-100. Sucks the fuel away pretty fast though.
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Old 11-25-2020, 07:30 PM   #11
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Quote:
Originally Posted by Ultra RV Products View Post
Start with a fresh MAF, it shouldnít struggle to reach 200 g/s. You can also use a baseline of what it reaches at the 1>2 gear shift at WOT. It should reach at least 230 g/s. Fuel trims at WOT arenít relevant as you go into open loop and the computer ignores the oxygen sensors.
Sound advice here as well
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Old 12-02-2020, 01:14 PM   #12
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I took the RV out for a bit of a run to find a hill today and took a data log.

Quote:
Originally Posted by Porkchopfat View Post
If your front oxygen sensors get up close to 900mv on wot you may have the beginning of clogged catalytic converters. To test for a pluged exhaust remove a front oxygen sensor and put in a vacuum/pressure guage. Anything over 1.5 psi indicates a clogged exhaust, most likely the catalytic converter.
The sensors have reached 900 once or twice, but not on WOT. On WOT they have read between 700 and 800mV.

Quote:
Originally Posted by Ultra RV Products View Post
Start with a fresh MAF, it shouldnít struggle to reach 200 g/s. You can also use a baseline of what it reaches at the 1>2 gear shift at WOT. It should reach at least 230 g/s. Fuel trims at WOT arenít relevant as you go into open loop and the computer ignores the oxygen sensors.
I didn't get a gear shift from 1>2 but did get 3>4 at WOT. Somewhat uphill and WOT for about 15s recorded between 159 and 185g/s over a 7 second period between 3,421 and 3,975rpm - so nowhere near the expected values.

Fuel trims are all over the place and appear to max out at 25 - regardless of WOT or not. They are generally pretty even across both banks, though I appear to have a small manifold leak at startup - so new headers next year I guess.

I had a friend who is more engine savvy come along, and he thought it was pinking on long uphill pulls - though the engine temperature never went too high. Hopefully, the new MAF and plugs will cure it.

If anyone is interested in the log, I can post it here.
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Old 12-10-2020, 11:10 AM   #13
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A further update:

Replaced the MAF, plugs and wires. As soon as the MAF was changed there was a notable improvement in how the engine sounded at idle. Not sure if the fueling was that far out but maybe it was?

No real issues removing the plugs and wires. Everything came apart OK and the existing plugs looked to be a pretty normal colour - though old and rusted in places. I suspect they are the original plugs!

I took it for a test drive along the same route I did before, using the throttle in the same places. There was definitely more power available, and the MAF reading was reaching up above 280 at the top of the rev range at WOT. Pulling up the main hill the MAF reading was about 30% higher than the previous test.

For the most part, the LTFT read between 5 and 6 on both banks. The average for the entire run was 5.38 on bank 1, and 5.60 on bank 2. This still seems off to me. At idle, the LTFT climbs up past 10 and 11, with bank 2 about 2-3 higher than bank 1.

The O2 sensors still move around pretty wildly, from very low to very high and back again! The average over the run was just under 0.5v across both, so I guess they are doing their job correctly to balance things out.

Any further thoughts on what I should look at next? I suppose I need to try and find any intake air leaks, and I already know I have a bit of a leaky manifold (drivers side), so maybe there's no point chasing until I get new headers?
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Old 12-10-2020, 12:37 PM   #14
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Sounds like you are within spec, which is +/-10% from 0. Oxygen sensor operation sounds normal.
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