1990 M103 economy gauge shows vacuum problem?
 

Hello: I did a search hereon regarding the economy gauge in my 1990 300SE with the M103 engine. I perused 17 pages to find questions and answers regarding the same. My understanding is that when at idle with the a/c off the needle should be pegged to the left. And that's the way it was until a few days ago.

Now, with the car in park, the engine idling and the a/c off, the needle is about 1/8 from the left. When I put the car in D and hold the brake, the needle moves to the right a little more for a total of about a quarter of the distance. With the a/c on, the car in park and idling, the needle is about 1/3 from the left. When I put the car in D and hold the brake with the a/c on the needle moves to the 1/2 mark or so.

If I'm driving the car and let off the accelerator the gauge pegs to the left. Oh, and the car sputters a few times upon hard acceleration.

What I'd like to know is where to begin checking for the problem. I didn't find enough information in the archives to tell me it's definitely this or that. For example, in some posts regarding the same problems, the OP was advised to check the oxygen sensor & others said the fuel filter. And, of course, many suggested a vacuum leak.

So, given my description of the symptoms I would appreciate advice on the best place to start. My car has 65k on the clock.

Thank you in advance.

Can someone confirm what the norm would be in all states?

-Idle, Park, AC off
-Idle, Park, AC on
-Idle, D, AC off
-Idle, D, AC on

I am also interested at what my E guage is telling me.

Are you idling high?

No, the idle is about 750.

One member had a leak at the cluster where the vac line attaches to the economy gauge. Mine holds far left even with AC on.

Yes, I read about those cases. However, it seems in some of those instances their needles weren't moving at all.

Is that in both P and D?

Park for sure. In drive, not sure... have to check.

Mine (89 190E 2.6, similar engine) is almost the same, except it's a bit more to the right than yours. I suspect a vacuum leak somewhere but I have not had the chance to look into it yet.

Vacuum lines under air mass unit have been replaced in July, I need to look into other lines.

My mother's 300TE is (well, was) nearly perfect in that sense. With AC on the gauge, when in P, was close to the left (1/8) but in D and foot on the brake it was more like 3/8 of the way, a few millimeters before going to the red zone.

I found this thread on proper position of that guage in different state. If the OP isn't experiencing these, I would think a vac leak check is in order.

http://www.peachparts.com/shopforum/tech-help/279895-economy-gauge-position.html?highlight=economy+gauge

I did read that thread before posting this one. I've concluded there is a problem somewhere, but would like help narrowing it down. Given the symptoms the car is displaying, especially the sputtering upon hard acceleration, I think someone with direct experience will be able to guide me to the most likely culprit. For example, if a faulty oxygen sensor does not present with the symptoms I've stated, then I can rule that out. If a somewhat clogged fuel filter does not present with the symptoms that I've stated, then I can rule that out....

That is high unless the A/C is on. In drive warmed up with A/C off at idle, should be about 500-550. Mine idles just barely over the 500rpm mark, and it goes up slightly with the A/C on.


On mine in park at idle the econogauge is full left. In drive it moves about 1.5 needlewidths off the full left, slightly more if the engine is cold and its idling higher still.

When accelerating, it should move to the right, but not go full pegged unless you're really on the pedal. Idling with A/C on it should be just left of the center when in drive, and will likely peg in the red when pulling away.

I'm in Southern FL, so I almost always have the a/c on. So, yes, the 750 idle is with the a/c on.

I'm certainly not a mechanic, but I believe one of the most telling symptoms of my problem is the car sputtering on hard acceleration, usually full throttle.

You should pull ~17" of vac at sea level with the M103... hot, in P/N. Which is basically pegged left (in the black).

D/R (or other gears) will decrease vac slightly.



Sputtering under hard acceleration is never good. Caps/rotors do not last long as the 103 tends to buzz pretty high down the freeway... same with plugs.

Maybe you received some poor fuel last fill up? Maybe the filter is restricting flow?

Go over the basics then choose your plan of attack.

Thank you all for your responses.

I will be changing the cap, rotors and wires soon. However, what I'd still like to know is, does the reading on my vacuum gauge and the sputtering strongly suggest one diagnosis over another? That is, perhaps the sputtering indicates a fuel and/or ignition problem. However, would a fuel and/or ignition problem have the effect on the economy gauge as I stated? I'm just trying to start with the most likely culprit instead of just throwing new parts at the problem.