Slight mis-fire at partial throttle...until I disconnect air temperature sensor

I have my 1973 450SL running better than it has in 20 years but noticed that under partial throttle there would be a slight mis-fire (arbitrary popping, not pinging).  Under full throttle everything was fine.  On a hunch I disconnected the air temperature sensor and it runs like a champ.  Very strong in virtually all situations I've run it under so far.  

If memory serves disconnecting the air temperature sensor forces the car to run richer.  Could it be running too lean otherwise?  Could I just have it a bit mis-timed for it to run that lean?

Thoughts? 

Google translated for my D-Jet friends over the big blue pond:
Ich habe meinen 450SL von 1973, der besser läuft als in 20 Jahren, aber ich habe bemerkt, dass es unter Teillast zu einer leichten Fehlzündung kommt (willkürliches Knallen, kein Ping). Unter Vollgas war alles in Ordnung. Aus einer Ahnung heraus habe ich den Lufttemperatursensor abgeklemmt und er läuft wie ein Champion. Sehr stark in praktisch allen Situationen, in denen ich es bisher gefahren bin.

Wenn der Speicher dient, zwingt das Trennen des Lufttemperatursensors das Auto, fetter zu laufen. Kann es sonst zu mager laufen? Könnte ich es nur ein bisschen falsch getimt haben, damit es so mager läuft?

Gedanken?

Hi Unknown,

yes it is true that mixture will run richer by roughly 20% (depends on ECU) if you disconnect air temp sensor. I would start by checking vacuum leakages on engine, fuel pressure and injectors.

The injectors were just re-built. The fuel pressure itself is fine, but I have not run the check on the volume (yet). I believe I've got a handle on any vacuum issues (my locks even work again!), but will review this site to see if there are any trouble spots I should focus on specifically. There's not that may spots, right off the engine, to go bad, but maybe I'll isolate the climate and locks to see if that is a contributor.

The darn thing runs so well with it unplugged.....but I'm trying not to cheat.

Hello,
and we still don't know your first name

Hans-Juergen

Russell Nile. The username I chose is something I use on many sites but it certainly doesn't look nice when trying to identify me as a real person.

I wanted to re-kindle this to get any fresh opinion's.  I have checked all vacuum points and they seem intact (will cut off right at the motor just to make triple sure).  I just replaced the fuel pump and have adjusted the pressure to just about 29.5.  It still pops and runs very rough with the temperature sensor connected, runs fine with it disconnected.

I ran a test against the sensor to check resistance at different temperatures and it seemed to be at spec, or at least within reasonable tolerances.

Thoughts?

 

Hi,

resistance does not change over vacuum. It is always the same. Transformation of input signals from ECU changes. That is why it is also called VLT - variable linear transformer.

Understood that the vacuum has no effect on the sensor. The testing I did was in different temperature water and I saw the resistance change. I matched that with a chart (that I likely found here ) and it seemed within reasonable spec.

The popping when I re-connect seems to be throughout the throttle, not just when slightly opened. It also seems to idle a bit more roughly. All of that goes away when I disconnect the incoming air temperature sensor on the air filter intake.

Hi,

that means you engine runs slightly too lean. Sensor removal increases mixture on warm engine by 10-20% . That is of course not a real solution. You need to find out what is wrong. Exhaust gas measurement per cyl might give a good hint.

I have the same problem with my Citroen DS 23 IE.
Exhaust CO is 2.8% with the air temperature sensor disconnected.

Fuel pressure is 30 PSI with engine running. With ignition only on, the highest fuel pressure is 22 PSI, even after a few on/off ignition cycles. The fuel pump is original to the car (51 years old).

I did turn the knob on the ECU 6 clicks clockwise, but that didn't make a difference.

Could it be a fuel volume issue?

As a follow up question...

Is the only systematic way to enrich the fuel mixture by turning the knob on the CEU clockwise?

Hi,

I do not consider turning rheostat on ECU by 6 clicks a systematic approach. Please remember that it only affects idle-run CO, nothing else.

It is mainly ECU and MAP sensor that really affect mixture. But they need sensors to reply correctly and they need a vacuum tight manifold, working injectors ...

Hi Samuel,
your car should have a fuel-filter that looks like a box. This should be replaced quite often...
Did you check it?

Regards
Norbert

I tested the MAP sensor (both vacuum and resistance) and it passed ok. I also dis a smoke test for leaks... none was found!

I have not tested the injectors, as it is a pain to remove them from my particular car (Citroen DS). But I would think that if there is a problem with the injectors it would manifest itself at all times, right?

Thank you Norbert,

Yes, I replaced the fuel filter last year, but the fuel pump is original and is now over 51 years old.

What are the options available for a replacement fuel pump?
Best
Samuel

I've been waiting until I get to a computer to properly respond.  Glad to see all the Jetronic interest! The people on this site have been incredibly helpful over the years.

I do not have the expertise of some of those that contribute, but I do have more years than I'd care to mention of trial and error on my 73 450SL.  Some thoughts about recent posts:

• As someone else pointed out, the computer dial only affects idle mixture so if your problem is above idle than it is something else.
• If you've got 51 years on a fuel pump I think it can be honorably discharged by now.  Keeping pressure is different than providing volume.  There is a test somewhere (xx ML in yy seconds) that may help prove if that is not the problem, but if a new fuel pump is available I don't think it would be a bad thing to replace.  That is a poke-and-hope, but I saw significant gains after a fuel pump replace (Bosch fuel pump, but it is a newer flavor....dunno if it will work for the Citroen, but if the JetTronic is the same I would think so?) 
• I've gotten my injectors re-built, but didn't see much of a change.
• I had my entire engine wiring harness re-built and that provided significant improvement.  If the computer is getting bad signals, for whatever reason, then it will respond badly. Dunno if that is an option for you.
• Many of the changes, combined, have significantly improved the operation.  The last time I had it out there was very little bad behavior and the temperature sensor was plugged in.  
• I'm still going to hunt for vacuum leaks.  While the tanks will hold for a while, so my door and trunk locks work now, but that doesn't mean there isn't a leak before the tank(s).
• I still have a horrendous hot-start problem.  It is notorious amongst the older 450's, but mine seems worse that I would expect.  Sometimes, I just leave my hood propped open when I park.
• - I'm still chasing gremlins...but they are smaller.

Hi,

I recommend to keep original Bosch D-Jetronic fuel pump as long as it works and does not leak and of course supply enough fuel. I do that myself. I would not follow our unknown friend x046866x. Please sign by firstname as it is usual here. And if you want to discuss your 450, please open a separate thread and let us not mix this one.

 

Apologies for weird username.  That was a poor choice when I registered and I'm contacting some folks that I hope can help me change that...,or I'll create a new one!

My name is Russell Nile.

The reason I chimed in is because I initiated this thread a while ago and when I received a recent notification I chimed in,  If it deserves to be a separate thread because of the difference in the vehicles then someone in the Citroen space will have to initiate that.  I assumed it came here because of a similar problem, but I am not familiar with the D-Jet used on the other vehicles.

Thanks for your help!

Hi Russell,

thanks for sharing your firstname. I was not aware that it had been your thread before. All I want to hint is that it makes sense to discuss both cars separately.

Actually, since I am the one who posted in response to Russell's post (because it was the same issue be it on another car).

In my mind, it makes sense to group similar problems together, since it is all D jetronic related. But happy to start a new thread if the group feels it is better.

Many thanks
Samuel