Talk Morgan Forums Morgan Sports Cars Maintenance & Servicing My continuing GDI saga, and its not good news.

Could the MAF have a hairline fracture that's giving flaky feed to the ECU when running (vibration) but static test the code reader sees it as ok ?

It could be that Jon. Certainly fuses can play with hairline cracks, and cause erratic faults, but usually a fault logs and pings the EML. Whether the GDI has a limp home remains unknown.

At least we now have a symptom, and cause. This second occurance recently though, was very gradual over distance, and only maxed out when we got home. After that it was constant. MAF cleaner had no effect, so perhaps a hairline opened up slowly. At home you just don't have the capability to substitute everything one at a time, to eliminate and close in on the cause.

At least I have a new fuel pump, and had the 'pleasure' of fitting that!

A few years back I was in Scotland with a pal and his +8 was refusing to behave itself. Called out the AA and the mechanic immediately removed and cleaned the MAF assembly and its connectors without even bothering to hook up a diagnostic.

He reckoned that the overwhelming majority of faults he encountered were MAF related so it was always his first port of call. He was right, the +8 immediately burbled into life and the hoon continued.

This is AI's search re MAF, AI also states that the Morgan GDI engine does have limp home mode (for what its worth) I can't imagine a vehicle not having a limp home mode these days to protect the engine. Does only your GDI not give you access to a reader, or are all GDI's in effect locked out. The reader element I left in, just in case others can read their error messages.


A Mass Air Flow (MAF) sensor might not generate Diagnostic Trouble Codes (DTCs) if it reports plausible-but-inaccurate data, has minor electrical or circuit faults, or is gradually degrading and the Engine Control Unit (ECU) is still compensating for its flawed readings.

Common Causes of MAF Sensors Not Generating CodesSensor Degradation:

The MAF element degrades slowly over time due to dirt, oil, or age. It sends incorrect air volume readings to the ECU, but because the signal is within the sensor's normal voltage range, no code is triggered.

Contamination: Oil residue from reusable performance air filters, road dust, and exhaust fumes can coat the delicate hot-wire sensor. It will misread the incoming air without tripping an open-circuit or high/low voltage code.

Air Leaks & Unmetered Air: Unmeasured air entering the engine after the MAF sensor (e.g., from cracked intake boots, failed PCV valves, or loose vacuum hoses) causes the engine to run lean. The MAF functions correctly, but the resulting O2 sensor or misfire codes will mask the underlying leak.

ECU Compensation: The ECU can adjust short-term and long-term fuel trims to compensate for a slightly failing MAF sensor. It artificially keeps the engine running smoothly, preventing an immediate Check Engine Light.Wiring & Connector Issues:

Corroded terminals, high resistance, or a loose harness can cause power drops or erratic signal spikes. If these issues do not cross the threshold into a permanent open or short circuit, the ECU will not log a code.

How to Diagnose a 'Silent' MAF IssueIf your vehicle is experiencing symptoms like poor fuel economy, rough idle, hesitation, or sluggish acceleration but lacks error codes, the issue can often be pinpointed using a live data scanner.Monitor Live Data: Connect an OBD-II scanner and read the "MAF in grams per second (g/s)"

. A general guideline is that a warm engine at idle should show a g/s reading roughly equal to the engine's displacement in liters (e.g., a 2.0L engine should read around 2.0 g/s).Inspect & Clean the Sensor: If the readings fluctuate wildly or are stuck at zero, inspect the wiring or carefully clean the sensor wires using only CRC Mass Air Flow Sensor Cleaner.

Never touch the delicate filament.Check for Unmetered Air: Use a smoke machine or inspect the entire intake tract between the MAF and the throttle body for cracks, loose clamps, and leaking vacuum lines.

Hi Dave
Great news on getting your car back up and running I am pleased for you. Like you I am an old distributor and carburettor man and used to fixing my own car so know how frustrating it is to have to hand it to someone else. But, as you say you can at least take comfort in knowing that without the resources available to BHM you would have ended up there anyway.

When I got my present Morgan I decided I would try and learn something about how this computer control stuff works and how to fault find with it. I found a book called “Automotive Diagnostic systems” written by Keith McCord. This is very informative and together with this, and bits and pieces on the internet got a better understanding of what goes on. The following is a bit of a simplification but basically as I understand it the computer has to deal with two things. (1) inject the right amount of fuel to get the correct mixture for different engine loads and RPM's, thus replacing the carburettor, (2 ) set the correct ignition timing thus replacing the distributor. It does this with a combination of referring to tables stored in its memory that we refer to as the engine map and measuring engine parameters with sensors.

In the case of the fuel mixture it starts by injecting the amount of fuel for a given rpm and engine load that is stored in the table but it has the ability to trim this value by as much as 25% in either direction leaner or richer. If it attempts to trim outside this range it will throw up an error code saying trim out of range and in what direction.

So with my basic understanding I have been trying to think up a story that would fit the facts in the problem that occurred in your case.

For what its worth here it is.

First there were no error codes and that must mean all the sensor values the computer was receiving must have been within their accepted ranges even if they were wrong.

John said that your MAF also contains the intake air temperature sensor or IAT so when the MAF was changed so was the IAT. The purpose of the MAF is to measure the weight or mass of the air entering the engine but what the computer really needs to know when trimming the fuel is the weight or mass of the oxygen contained within that air mass. That will vary with air density the hotter the air the less dens and the less oxygen. To calculate the density the computer needs to know the intake air temperature. Suppose your IAT was within range but was reading higher than it should,. that would be interpreted as the air being less dens than it actually is and cause the fuel trimming to reduce the amount of fuel injected and you would be running too lean. Also the hotter the air the hotter the engine and to protect against detonation the computer will retard the ignition. As you drive along the engine bay will of course get hotter and this will increase the intake air temperature making the situation even worse this is in keeping with what you experienced. Under these conditions you would be running too lean and with ignition too far retarded and no error codes because every thing was in range. Remember this is my fairy story with absolutely no experience of chasing these faults but based on what I have read but may have miss interpreted.

I am still trying to learn more about this subject so if anybody reading this can find holes in it I would love to hear.

By the way, following the progress of Monty and his deep breath on this forum I have found it to be quite educational in regards to engine mapping and computer control. Thank you Montegue.



Sorry about the long post


Regards Tony

In answer to are all GDIs locked out the answer is no. I had one of the first GDIs from new, a 2014 car, that used to regularly display the engine warning light when applying full throttle after a lengthy time at low revs. So stuck in traffic and finally having an overtaking opportunity I could almost guarantee the light would appear. I used to carry an OBD reader with me so I could clear it when I stopped. No idea if Morgan locked later cars.

Here are some stats.

First MOT in June 2019:
2500/3000rpm.
CO%vol 0.10 Max 0.20
HCppm 11 Max 200
Idle
CO%vol 0.285 Max 0.30
Lambda 0.998 Range 0.97 - 1.03

2025 MOT
Failed AR flash
2500/3000rpm.
CO%vol 0.516 Max 0.20
HCppm 441 Max 200
Idle
CO%vol 0.03 Max 0.30
Lambda 0.994 Range 0.97 - 1.03

2025 MOT
Pass with standard map
2500/3000rpm.
CO%vol 0.019 Max 0.20
HCppm 9 Max 200
Idle
CO%vol 0.264 Max 0.30
Lambda 1.003 Range 0.97 - 1.03

New MOT. New MAF:
2500/3000rpm.
CO%vol 0.07 Max 0.20
HCppm 25 Max 200
Idle
CO%vol 0.17 Max 0.30
Lambda 1.000 Range 0.97 - 1.03

Dave just out of interest I see your tester is using a generic limit for the lambda.

Page 126 of the 18th edition



Not trying to be a smart Alec, but mine is always tight on the lambda with a k&n filter fitted. I take a copy of the emission book with me, just in case.

I'm not trying to be funny here but if the scanner even if not locked out couldn't find a fault code what does the EML element record? My past history with modern cars has shown me that the MAF faults are the first to register even slightly out of normal function parameters even if still within range. Your car was clearly way outside of this and was this way from new yet Morgan were unable to find it. Even BHM clearly couldn't find a fault when scanning your EMS this time and I'd guess they had viewed live scanning as well..... if they can? I think that your history of your journey trying to solve this problem allowed BHM to finally follow an accelerated fault path of known issues with the Gdi and led them reasonably quickly to the MAF sensor, so well done Dave you had a very relevant part in this diagnosis too!

Now my engine is the original Duratec and I'm not locked out of the OBD2 port but what you can interrogate quite frankly is next to worthless so I guess I wouldn't have been able to have done any better than you on the Gdi. I'm sure I would have covered the same checks on the MAF system, mechanically and electrically as you did and struck it off the list of potential faults....... looks like we may need to hold a known master set of components in order to carry out our future checks. Lastly, I can't see why Morgan would lock access to the OBD2 port. Firstly its illegal, and I cannot believe that after all this time a bypass solution wouldn't have leaked out, BMW can't keep people from accessing their systems for very long yet the Gdi has been around for 12 years! If my plus 4 is anything to go by I think the OBD2 port has little to offer in the way of diagnosis. Still it has done one thing Dave......... I'm sure we will all look far more closely at the MAF in future and I'd probably group the throttle position sensor in that as well!

I've used that list for my Roadster IcePack, but it doesn't list the GDI, so I take from that that the generic is the relevant range.

Sewin you nailed it. BHM have never encountered this problem and so in the future, will look at the MAF sooner. I certainly will, and if this now happens to any other GDI, they have a head start, because it might just be that changing the MAF resolves it immediately.

Hindsight eh? If only I'd known.