Hello everyone, let me introduce myself: I’m writing to you from Spain (sorry for my English) and I have a 1994 Mercedes S500 C140 (W140 Coupe) 5.0 V8.
I know this forum is dedicated to the E500, but I decided to write anyway since I’ve been reading the forum for a while and found a lot of useful information.
As I mentioned, my S500 is from early 1994. It has a 119.970 engine with LH injection. I’ve had it since 2009 and used it relatively frequently until 2016, and since then its use has been very sporadic, but it has always worked perfectly.
During that time, it had minor issues typical of this model that were corrected without much difficulty.
The problem I'm facing now is driving me crazy because I don’t know where to start. Here’s what’s happening: It starts on the first try without effort, but instantly begins to misfire dramatically, as if half of the cylinders have ignition problems, with a lot of smoke from the exhaust, sometimes black, and a huge smell of fuel—it's actually impossible to breathe behind it. The engine stutters and struggles to rev in the initial part of the throttle. Although it can move, it does so with jolts and stutters, making it impossible to drive on the road.
At first, I suspected the usual issues: spark plugs, distributor, cables, coils, etc., even though many of these parts were changed in recent years. To my surprise, I discovered through testing that if I disconnected the MAF, the car would run perfectly as if by magic, with amazing smoothness, but it would stall as soon as I plugged it back in. I decided to look for another MAF and ordered one on eBay, supposedly tested and functional.
As soon as it arrived, I installed it, and to my dismay, the problem continued. I then decided to replace the lambda sensor and the coolant temperature sensor, but nothing changed. Only once, during countless tests, I started the engine with the temperature sensor disconnected, and mysteriously it ran well—even after reconnecting it, it started fine 3 or 4 more times. But never again.
I’m ruling out (not sure if correctly) any mechanical problems since with the MAF disconnected, the engine runs perfectly, or at least with very little difference compared to how it used to. Therefore, I’m focusing on the electronic control of the injection—maybe I’m mistaken?
I’ve checked the MAF voltages, its power and ground, and they are correct. I’ve checked the fuel pressure to rule out the pressure regulator, and the LH module does not store any faults (I have a Bosch KTS, but unfortunately don’t have a Star).
What do you think? Where do you suggest I keep looking? Could it be that what’s happening is that by disconnecting the MAF, the LH module is ignoring some faulty values/sensors (I don’t really know which ones) and substituting them with defaults, which is why it runs well?
Am I correct in ruling out potential causes like spark plugs, cables, distributors, EZL module, etc., and focusing only on the sensors and wiring of the fuel injection components?
I’m really lost—why on earth does it run so well without the MAF and so poorly with it? Could the “new” MAF be faulty while giving the correct voltage readings?
By the way, although it runs well without the MAF, the CO values are completely off the charts and lambda is around 0.700. The mixture it’s making is very rich. I wouldn’t be surprised if the catalytic converters are completely dead.
Thank you in advance for any guidance or support you can provide; it’s greatly appreciated!
I know this forum is dedicated to the E500, but I decided to write anyway since I’ve been reading the forum for a while and found a lot of useful information.
As I mentioned, my S500 is from early 1994. It has a 119.970 engine with LH injection. I’ve had it since 2009 and used it relatively frequently until 2016, and since then its use has been very sporadic, but it has always worked perfectly.
During that time, it had minor issues typical of this model that were corrected without much difficulty.
The problem I'm facing now is driving me crazy because I don’t know where to start. Here’s what’s happening: It starts on the first try without effort, but instantly begins to misfire dramatically, as if half of the cylinders have ignition problems, with a lot of smoke from the exhaust, sometimes black, and a huge smell of fuel—it's actually impossible to breathe behind it. The engine stutters and struggles to rev in the initial part of the throttle. Although it can move, it does so with jolts and stutters, making it impossible to drive on the road.
At first, I suspected the usual issues: spark plugs, distributor, cables, coils, etc., even though many of these parts were changed in recent years. To my surprise, I discovered through testing that if I disconnected the MAF, the car would run perfectly as if by magic, with amazing smoothness, but it would stall as soon as I plugged it back in. I decided to look for another MAF and ordered one on eBay, supposedly tested and functional.
As soon as it arrived, I installed it, and to my dismay, the problem continued. I then decided to replace the lambda sensor and the coolant temperature sensor, but nothing changed. Only once, during countless tests, I started the engine with the temperature sensor disconnected, and mysteriously it ran well—even after reconnecting it, it started fine 3 or 4 more times. But never again.
I’m ruling out (not sure if correctly) any mechanical problems since with the MAF disconnected, the engine runs perfectly, or at least with very little difference compared to how it used to. Therefore, I’m focusing on the electronic control of the injection—maybe I’m mistaken?
I’ve checked the MAF voltages, its power and ground, and they are correct. I’ve checked the fuel pressure to rule out the pressure regulator, and the LH module does not store any faults (I have a Bosch KTS, but unfortunately don’t have a Star).
What do you think? Where do you suggest I keep looking? Could it be that what’s happening is that by disconnecting the MAF, the LH module is ignoring some faulty values/sensors (I don’t really know which ones) and substituting them with defaults, which is why it runs well?
Am I correct in ruling out potential causes like spark plugs, cables, distributors, EZL module, etc., and focusing only on the sensors and wiring of the fuel injection components?
I’m really lost—why on earth does it run so well without the MAF and so poorly with it? Could the “new” MAF be faulty while giving the correct voltage readings?
By the way, although it runs well without the MAF, the CO values are completely off the charts and lambda is around 0.700. The mixture it’s making is very rich. I wouldn’t be surprised if the catalytic converters are completely dead.
Thank you in advance for any guidance or support you can provide; it’s greatly appreciated!
