Hesitates under load when hot

[markko]

Hi everyone. I have a 78 with DFEV carburetor and upgraded electronic ignition. I am also using the Malpassi Filter King fuel filter and pressure regulator. The car has been running beautifully, at least until the last few days. With the arrival of summer temperatures here in the Great White North (30C or about 90F for you non-metric people) I have noticed that when when the engine is hot and is accelerating under load (for example, up a hill), it hesitates and bucks, at times quite badly. I replaced the fuel filter, which had fixed a similar problem once before. I'm thinking that it may be the gas. Normally, I would use ethanol-free premium, especially when storing for the winter, but the last few fills, I have used regular with ethanol. Could this be the issue? From looking around the forum, it seems that it may be, and I will fill up with ethanol-free gas and see what happens. Any other thoughts about this? As I said, she was purring like a kitten until very recently. Thanks.

[18Fiatsandcounting]

...but the last few fills, I have used regular with ethanol. Could this be the issue?

Possibly, and a refill with different gas would tell you pretty quickly if this was an issue. However, I'm thinking that there is something else going on, and running regular/ethanol gas just "pushes it over the edge."

"Hesitating and bucking under load" sounds to me like a misfire, and the first places I would look are the spark plugs and plug wires, followed by the coil and electronic ignition system. Some electronic ignitions work just fine on the Fiats, but if you search this forum, you will find that many do not. Another possibility is that the engine is running very lean under certain conditions. How do the spark plugs look in terms of color and residues? Oil fouling can also cause a plug to misfire.

-Bryan

[Nut124]

I would install a fuel pressure line in a T before the carb and run it under the hood and attach to wiper so you can see it while driving. Could be low fuel pressure that shows up when you call for power.

What kind of a fuel pump? Many regulators hold pressure nice at idle but drop down under load. Is the regulator in the trunk or in the engine compt?

[markko]

The plugs look good. I have the original mechanical fuel pump, and the regulator is in the engine compartment. What is strange (and maybe coincidental) is that this coincided with hot weather. I will play with the fuel pressure regulator and switch out the ignition components and see what happens. It does feel like a fuel issue though. When it starts to buck, I can make it smooth out by easing off the gas or changing to a higher gear. It seems to kick in at about 3500-4000 RPM.

[aj81spider]

Why do you have a pressure regulator with the original mechanical fuel pump? The pump is sized correctly for the car and originally the engine didn't come with a regulator. My guess is that the regulator is limiting the fuel when you have high demand.

I have always used regular with ethanol on the two Fiat's I have owned (that's all that's readily available here) and have never had an issue with it, so I'd be surprised if that was your problem (although I've been surprised before).

The first thing I'd try is removing the regulator and seeing if that solves your problem.

[18Fiatsandcounting]

Try full throttle acceleration in 3rd gear (no shifting) from 25 mph to 65 or so, perhaps up a slight incline. This test calls for a lot of gas, lasts long enough (10 seconds or so) to see if there is a fuel delivery issue, and keeps things safe and legal.

If there is no hesitation or bucking with this test, it wouldn't seem like a fuel issue.

I do agree that you don't need a regulator with a mechanical fuel pump. One of the benefits of a mechanical pump is that more fuel is delivered the faster the engine runs, in contrast to an electric pump that delivers the same amount of fuel no matter what the engine needs.

-Bryan

[markko]

Thanks for your help everyone. I have a pressure regulator on the advice of mechanic I had taken it to last year who said that the pressure coming from the pump was too high. Anyways, it turns out it was a fuel pressure issue. I put the gauge on, and it read less than 1 PSI. I turned it up to about 2.5, and took her for a long drive with no issues at all. It's great to have such community support. Thanks again.

[Donstoy]

Hi
I have a 1980 Spider 2000 with fuel injection and I have experienced a similar problem with what appears to be a sudden fuel deficiency after driving for about 40 km. The car will suddenly start to lose power and “stagger” as if there was a sudden blockage in the fuel delivery to the engine. We have changed the fuel pump and filter, and cleaned out the fuel lines and the vent from the tank but the problem keeps recurring. When this occurs, I pull over and turn the car off for 5 minutes or so. When I restart, I can drive normally for 10 to 15 km and then it will happen again. I am using 91 octane gas. Any thoughts?
Don

[RRoller123]

Try a vented gas cap, temporarily, if the problem goes away, then you will know that it is in the evap control system, charcoal canister and lines. In the FI cars, that is the only way that makeup air gets into the fuel tank.

[Donstoy]

Thanks. I did not have a vented gas cap so I tried driving with the regular cap loosened to allow air to enter through the cap area. I secured the cap so that it would not vibrate and come off entirely. The result was that I was able to drive about 70 km before the chugging and loss of power returned. This is almost twice the distance that I drove yesterday before the problem surfaced. A definite improvement but not a cure. Any further thoughts?
Don

[RRoller123]

hmmmm..... that may be a positive indicator. Here is the system:



Note that the Carbed cars are VENTED GAS CAPS, the FI cars are SEALED GAS CAPS.

[18Fiatsandcounting]

Thanks. I did not have a vented gas cap so I tried driving with the regular cap loosened to allow air to enter through the cap area. I secured the cap so that it would not vibrate and come off entirely. The result was that I was able to drive about 70 km before the chugging and loss of power returned. This is almost twice the distance that I drove yesterday before the problem surfaced. A definite improvement but not a cure. Any further thoughts?
Don

Don, perhaps the problem is not fuel related? I have almost zero experience with the "younger" spiders, but I have heard of problems with either the coil or the ignition control module, when the engine gets really warmed up. That is, running more than 15-20 minutes or so.

Misfiring in the ignition system would also cause the symptoms you're describing.

-Bryan

[aj81spider]

It's a bit of a long shot, but one other thing to check is the air hose that goes from the air filter box to the intake plenum. I had a problem where my '81 fuel injected car would lose power and surge after driving for 20 minutes or so. It turned out to be a crack in the rubber that only opened up after I'd been driving a while.

The symptoms don't match you perfectly, but it's easy to check.

[Donstoy]

Thanks guys. I will follow up on these ideas and hopefully resolve this issue so I can get back on the road without worrying about how far I will be able to go before she shuts down on me.
Don

[RRoller123]

Check that small diameter section of hose where the big tube branches over to the AAV, it is particularly susceptible to cracking.