Got diagnostics now re my running issue but no good known MAF

[silver50]

1994 SL500. If you helpful folks here could cast your eyes on my MAF running data just for confirmation that’d be good.

Can I ask: (this was mentioned on here) How can an MAF show as faulty but might not be? (I understand live data being the determining factor but don’t understand why one would show up as a fault if possibly operating ok)

I mentioned on a separate thread my original Bosch MAF has gone missing while the car was at a repair shop. It wasn’t one with lots of miles on it either. My MAF was determined as faulty by the shop but I don’t think live data was observed. The loss of the part is still to be ironed out with the shop owner. I am in the UK and just cannot easily locate a known good used MAF 0000940148 at a reasonable price. If anyone can help me I’d be grateful.

To recap, my ‘94 SL has been a big revival project doing a major underbody restoration. It wasn’t a known quantity to me regarding running/past use. 72,000 miles. I’ve replaced the upper wiring loom and ETA wiring is re-done. New coils, leads, caps, rotors, plugs and FPR. It starts well, settles fine to idle but runs rich and hesitates on blipping throttle. Plugs look sooty. Engine shaking slightly.

I should have got myself equipped with diagnostics but opted to take it to a shop. Also to tie up some jobs after my work, like alignment.

I have the car back and someone I know located his old diagnostic laptop. The only engine fault showing is MAF. The one now on my car looks like used/unbranded. I know from on here the readings should be approx 30 kg/h-100kg/h. Mine seems pegged at 1563.5 kg/h. This (1563.5) screen below is engine running. The other screen is MAF disconnected.

Would those here agree that the module appears to be getting readings ok and that there’s a fighting chance a good MAF would help cure my issues?

The car has not had a good run in years. I can now drive it but is there any sense in this with the rich running, probably not? I believe the guy at the repair shop re-set LH adaptations.

Thanks v much for help. I’ve done a lot of work to the car so am keen to resolve this and actually start using it!

 

[gsxr]

Based on those readings... it does look like the MAF currently installed is defective. At 650rpm idle the airflow should be around 18-22kg/hr at operating temp (may read closer to 30 after a cold start, during warmup phase). Screenshot below from my car at hot idle, video here.

Now, since the LH module is what receives the MAF signal, there's a tiny chance the LH module could be part of the problem, but that is extremely unlikely. I am assuming your upper engine harness was either replaced with new OE, or professionally rebuilt, and the harness has been ruled out as a potential issue.

In the meantime, the car should be drivable with the MAF disconnected, but it will not make normal power or have normal fuel economy.

 

[silver50]

Many thanks for the input and video/screenshot. Sounds hopeful an MAF might solve things. Do my MAF disconnected readings look as you would expect at 1.20v and 4 kg/h? I was hopeful those possibly indicate the LH module side of things is probably ok.

Yes, my harness and ETA wiring was professionally re-done but I haven’t done any harness checking.

Is there any point in my buying a cheap MAF to pop on the car? I’m puzzled as to why they don’t seem to be an option at all.

 

[R129 UK]

Now, since the LH module is what receives the MAF signal, there's a tiny chance the LH module could be part of the problem, but that is extremely unlikely. I am assuming your upper engine harness was either replaced with new OE, or professionally rebuilt, and the harness has been ruled out as a potential issue.

For my own understanding, is the Hot Wire Voltage in HHT-Win the actual value direct from the MAF or is it what the control module is interpreting or calculating itself and using to control the engine?

Is the air mass calculated from the hot wire value?

From past non-Automotive applications, I thought hot wire anemometers used current not voltage or is the current used to calculate the mass and the voltage is an indicator that the device is operating electrically?

Sorry for all the questions

 

[gsxr]

Many thanks for the input and video/screenshot. Sounds hopeful an MAF might solve things. Do my MAF disconnected readings look as you would expect at 1.20v and 4 kg/h? I was hopeful those possibly indicate the LH module side of things is probably ok.

Good question. I've never paid attention to what the live data shows with MAF disconnected. I need to try that and document the results.

Yes, my harness and ETA wiring was professionally re-done but I haven’t done any harness checking.

Is there any point in my buying a cheap MAF to pop on the car? I’m puzzled as to why they don’t seem to be an option at all.

At this point I'd look for a good used MAF with a return policy, ideally located in UK. I'm not sure why the Chinese haven't copied these yet to sell cheap... possibly not enough demand, as the unit is specific to M119 V8 engines with LH-SFI injection, and there weren't a ton of those produced. If you can't find one locally, drop me a line.

For my own understanding, is the Hot Wire Voltage in HHT-Win the actual value direct from the MAF or is it what the control module is interpreting or calculating itself and using to control the engine? Is the air mass calculated from the hot wire value?

From past non-Automotive applications, I thought hot wire anemometers used current not voltage or is the current used to calculate the mass and the voltage is an indicator that the device is operating electrically? Sorry for all the questions

All I know is that the hot wire voltage shown increases proportionally with airflow, but is usually a minimum of 1.20 volts, and shows ~1.50v at hot idle. Click here for a Bosch technical PDF that may answer more of your questions, @R129 UK .

 

[R129 UK]

All I know is that the hot wire voltage shown increases proportionally with airflow, but is usually a minimum of 1.20 volts, and shows ~1.50v at hot idle. Click here for a Bosch technical PDF that may answer more of your questions, @R129 UK .

Thanks Dave, that is a really useful document and, as I suspected, it is the current that determines the quantity of air passing the hot wire:

 

[silver50]

Interesting stuff. I see the kg/h range level on our V8 sensor seems to go up to a surprisingly high 950 kg/h. The sensor listed before it seems to be the Volvo one, up to 650 kg/h. This being with my understanding the working range is up to around 100.

 

[R129 UK]

The part number ending in 004 is what you are after I believe, the range is greater due to the volume of combustion air needed on a large capacity V8 engine:



If I understand you correctly...

The graph shows the 004 unit can operate up to ~1200 kg/h:

 

[silver50]

Yes the 004 is the Bosch A0000940148. It was the range going away up to as high as that that seemed surprising. I thought the range with the engine running in use was only roughly 30-100 kg/h.

 

[R129 UK]

I'm not a fan of AI but this seems pretty accurate:



Assuming it is a linear curve, peak RPM would be about 200 kg/hr.

 

[R129 UK]

I've just re-read it and those numbers are at idle so under load they would be considerably higher.

A further edit... the link next to the 3000 rpm text links back to a thread on here!!

Thread 'What are normal Mass Airflow Sensor (MAF) readings in live data?'

Feb 17, 2020

Thanks Dave - I will test both MAFs with Star in my 500E tomorrow and if they check out fine I will order up 2x screens

hello there, im sorry for posting here again
can u please tell me what it should read on star live data? mine reads 32, whats the best wat to teat it using star?
thanks in advance

• aliking
• Replies: 11
• Forum: Troubleshooting and Diagnostics

• 

The link above also links to this thread which is also an interesting read:

FYI Post in thread 'M119 MAF Cleaning'

Jul 9, 2019

PS the E500E chassis does perform it’s own MAF cleaning after certain conditions – see the image attached.



I did replicate the 10x starts and min RPM each time but this made no perceivable difference to these MAF s prior to manually cleaning them with the special spray. But assuming they are not very dirty with 30 years of service on them – this hot wire burn off system should keep them serviceable for years after.

• JC220

• 

 

[gsxr]

I've just re-read it and those numbers are at idle so under load they would be considerably higher.

A further edit... the link next to the 3000 rpm text links back to a thread on here!!

LOL... that's an AI summary of my old posts! Readings of 20kg idle, and 30/60/90 at 1k/2k/3k RPM are numbers I've posted several times. And yes, those are measured at no load, revving the engine in Park. Oddly, the readings are the same regardless of displacement (4.2, 5.0, 6.0).

Airflow readings under load while driving are much higher, say 200-500 kg/hr, varies with engine displacement, load, altitude, etc. I want to say a 6L engine is pulling around the 900 range at WOT but I'd need to double check that.

 

[R129 UK]