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Loss of power when hot

4K views 13 replies 4 participants last post by  MBGraham 
#1 · (Edited)
Our 72 350SL gets used mostly on shorter trips in our semi-rural area.

Yesterday, we took the car on a longer trip to Manotick, Ontario and back via back roads. About 150 mile return trip. (MBCA event - 6 of 13 cars were 107s! (1-350, 1-450, 1-500, 4-560s)

Car ran well but I did notice that towards the end of each leg, the car lacked it's usual zip. Floor the pedal, and engine only slowly responds. It was a hot day (for us) over 30C (86F) but coolant temperature was steady at a little over the 175F marking - maybe 190F?). So puzzling why this happens. Anyone else have similar symptoms?

For first time in years, I checked fuel consumption. Not so good at 10.5mpg of 90% highway driving at 60-65 mph. The AFR gauge read in the 14-14.5 range when cruising on fairly flat roads. This is higher than it used to be (About 13.8). Fuel pressure was normal. Might be sucking in some air into exhaust (O2 sensor is just before mid muffler) or maybe injectors are not providing full capacity. But why only when hot?

Under hood temperature was probably higher than normal. Maybe some of the Djet components are affected by temperature? Or maybe the engine is just getting old (like owner!) 160k miles at present.
 
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#3 ·
Both temperature sensors are relatively new. No change in altitude.

Possibly vacuum leak that gets worse with temperature?

or

Actual temperature on long run is say 10deg higher than temperature I used to set mixture (and that car normally runs at around town). Causes incorrect mixture.

Pertronix or Switchgear affected by temperature and not providing same intensity of spark?

But just guessing.

Took car for run this evening. Back to it's normal self. Tested on same hill it laboured on at end of 75 mile run. No problems. Gave it a bit of an Italian tune-up while I was about it :)

Probably no easy fix or answer. Probably have to check and/or swap everything :( Then go on another 150 mile trip :eek
 
#6 ·
I was using 10% ethanol fuel (Our Reg/mid grades are E10). I started with full tank. Noticed slight lack of power after 75 mile run. We stopped for about 6 hours. On run back, I noticed engine was less lively as we got closer to home. I wondered if low fuel might make a difference, so filled up. After that we have a small hill where I sort of know how car usually performs. It was labouring when going up that hill with full tank.

Car runs fine in town. But AFR says I am running leaner than when I originally set car up. Maybe I get away with that around town, but when engine is say, 10F hotter, the mixture gets leaner still and then lacks power. As you know these engines like to run rich.

Probably change fuel filter, swap injectors for spare set and see what happens. But too hot here at present to mess with car :givemebeer:
 
#5 ·
What happens to air and fuel at higher temps?

Fuel might bubble (lean condition)

Air expands becoming thinner (rich condition).

You mention low MPG (rich?)

You mention higher AFR (lean?)

I am thinking that some more testing is in order as your data seems to be pointing you in two opposite directions.

Is it only under throttle that you are disappointed? Checked / cleaned the tank strainer recently?
 
#7 ·
Thanks for ideas

What happens to air and fuel at higher temps?

Fuel might bubble (lean condition)

Air expands becoming thinner (rich condition).
My fuel pressure is normally about 32psig and gauge said it was actually higher 34-35psig. But gauge is not too accurate - need a better one. But should not get fuel vaporization in rails while driving at speed.

You mention low MPG (rich?)

You mention higher AFR (lean?)

I am thinking that some more testing is in order as your data seems to be pointing you in two opposite directions.
You are right that those conflict. Possible explanations:

- Because of lack of power caused by overly lean mixture, engine runs less efficiently and ends up actually using more gas? Does that make sense??

- Possible vacuum or exhaust system leak causing incorrect AFR at mid muffler.

Is it only under throttle that you are disappointed? Checked / cleaned the tank strainer recently?
That is when I mostly notice it, but once in a while at cruise speed, I get a sensation similar to if I had momentarily taken foot off pedal, when in fact I haven't. So short, it's hardly noticeable. This even in town.

Don't want to think about tank strainer :( Changed fuel filter 12k miles ago. Probably should change it and put in my new fuel pump whether cars needs them or not! At same time could check gravity flow rate from tank strainer.

One other piece of data. Our normal summer temperatures are in mid 70s, but recently and on trip they have been in high 80s.
 
#9 ·
I was thinking that this might be a good time to install the Flamethrower coil that I have never got around to installing and bypass the switchgear. But thought also passed my mind that it could be the Pert itself that is heat sensitive.

As usual no simple answers and hard to do test.

Job list so far - new fuel filter and pump; Swap injectors; Flamethrower install; check and change plugs; re-adjust MPS with car really warmed up; new air filter; check vacuum; check exhaust for leaks.

May not get to all these, but nice to have a list! (I left off the tank strainer ;) )
 
#10 · (Edited)
As posted in another thread relating to hot starts, I just got back from a similar run to last years, except to a different small town. (Perth, Ontario) - Interesting small historic town. MBCA Ottawa had arranged for Town Crier to take us on a walking tour. Quite interesting. Followed by nice lunch at German restaurant. Drive there and back was on country roads - 50-60mph.

Return distance and type of roads were about same as last years trip. 150 mile return trip. Car ran really well. No loss of power like last year and better fuel consumption (15mpg). I had replaced a lot of things including all harness connectors, trigger points, plugs, distr cap & rotor. Did not adjust timing or mixture or change any sensors.

Don't really know what main reason for better running is. Perhaps combination of things.
 
#13 ·
Wow - Why am I surprised :)

Not having the 3&4 injector plugs reversed probably also helps.

My car has always run quite well - at least since I put in the wideband and got the AFR adjusted. But this is the smoothest I can recall it ever running.

New tires tomorrow :thumbsup:, then try and fix odometer :(
 
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