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Discussion Starter #1
Hoping someone has a suggestion. My 450 sl runs great. But as soon as I reach 80 mph the engine stumbles. Never before 80. I can cruise at 78mph all day???? All fluids are at proper levels. Tranny shifts correctly.
Is there an engine governor to control speed on a 1977? Are there any other causes I haven't thought of? Any help would be appreciated.
 

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Have you checked the fuel pump howe about the gas filters? I had a 1978 Jaguar sedan that would sputter at high speed.
A new fuel filter fixed the problem.
 

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Reminds me of a situation I used to see back when everyone ran carburetors. The higher cylinder pressure at increasing speed due to load makes it harder for the spark to jump the gap on the plugs due to carbon deposits. So items to check would be the spark plugs, cap, rotor and wires. Id look at the distributor cap and rotor first and check if they clean and dry with no cracks. Then check the plugs for deposits and correct gap. Wires could also be an issue but you would need to put a meter on them and check resistance to make sure its in spec.
 

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You should test it in other ways to help you/us figure it out. Like is it just 80 or 80 an above? If you drop down one gear does it do the same thing at the same speed, which I'm guessing it will not. Does it do it under acceleration through 80 or only cruise? Like start at 75 and give it a lot of gas, but not so much that it downshifts. And at 80 while it's doing it, give it gas until it stops doing it and note about how much gas it took relative to full throttle. Does it do it accelerating in other gears or cruising in other gears. If you have hills where you live see if it does it going up/down at 80. Basically I/we want to see where else it does it to more easily figure out what to look for. My first test with any car would be to floor it through a couple gears to see how it runs through the rpm band under max load which usually forces the problem to appear. Then play with it to see if it was rpm or throttle position that causes it. I say throttle position but really I'm looking for intake pressure, so without a vac gauge you just have to estimate. So say half throttle at 1500rpm is high intake pressure, but half at 6000 is not. Probably TMI but the point is try every rpm, load an speed to narrow down exactly what conditions it does it. Like say you can floor it from a stop and it peters out at 2k rpm, but at say half throttle it's 3k, 1/4 throttle it's 4k etc, this would point to fuel delivery so I'd look at the pump and filter. If only rpm related, like it goes to 2500 and that's it no matter the throttle pos, then it's no doubt an electronic controller tell it to do that or the electronics are failing at that point.
So saying it does it at 80 is just one piece of the puzzle so I/we need more pieces to figure out the picture on top ;)
If it were spark then you'd no doubt see it elsewhere many times during normal driving because the load and rpm at 80 is very light and you should hit that load all the time under normal driving. Well, I would but my grandma literally told me she drives like there's an egg between her foot and gas pedal.
Other than motorcycles with stator powered coils I've yet to see a spark so weak it does does what you describe. Car coils usually just stop working altogether and may or may not come back online when they cool down, but they're generally an On/Off fail. Things like dirty plugs, shorts in the cap/wires can probably cause what you describe but I've never seen it. What I see is they do it randomly yet consistently. Like one hole will be missing all or most of the time, never only at a certain speed. I've had electronic ignition modules that will drop out at a certain rpm. Which they will do when new but as they give up the ghost that rpm gets lower and lower, like an rpm limiter that some Gremlin is turning down randomly. I recall this especially with GM modules from the 80's (I think) which drove me nuts and forced me to buy aftermarket and tell the same to everyone with that issue.
So my first assumption is fuel delivery, which will show up in the aforementioned testing. So the fuel filter would be my first suspect and if it hasn't been changed in the past 50k or so I'd change it anyway.
Same with the ign sys because it certainly never hurts to throw on new cap/rotor wires & plugs, which it may be due for anyway? I change plugs a lot more than the others, and usually change wires a lot more than caps, but I know what to look for, how/why they fail and how to make them last longer. It's much easier to replace them all than learn that. Just buy good quality parts or you may be adding a new problem or will have a new problem much sooner than you might expect. Fyi I hear there are a lot of counterfeit spark plugs out there, specifically the expensive ones. In a car like yours I wouldn't bother with special plugs like iridium, platinum or any other bs plugs, just get cheaper regular ones. Autolite is a fav of mine because they're excellent yet cheap. The reason is the #1 failure for plugs in a car from your era is fouling, not wear, and the expensive plugs foul just as fast and do so long before a cheap one will wear. So why buy a plug that could last 200k miles but fouls out much sooner? So it's much better to have new (clean) plugs than any kind of fancy bs snake oil plugs :)
 

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Discussion Starter #6
Thanks everyone for your suggestions. Here's an update. Decided to change the fuel filter as the first step. I found the filter was installed backwards. I replaced the all of the braided fuel lines at the same time. I now have no power to the pump. Maybe a fried relay? On order and will update after relay is replaced. While I have in the air I'm doing a bunch of other maintenance that does not appear to have been done over the last few years. Plugs/cap/rotor /brake lines etc will be part of it. Will update after all complete.
 

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Discussion Starter #7
Help. I replaced the fuel relay. No difference. I even ran a jumper from the battery to the fuel pump to make sure pump didn't fail. Pump runs when jumped. Car turns over but will not start.
What could I possibly be missing? BTW, after having the key in run position for a few minutes I could feel the relay and it was a little warm....not hot. seems like there is power to the relay.
 
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