1980 Fiat spider 124 running rough after long warmup.

[Truthhurts21]

I have a Fiat spider 124 that is running rough after about a 15 or 20 minute warm up. I just replaced the fuel tank which probably could have been cleaned out and would have been fine. I also replace the fuel filter. This was what owner wanted as she was told by another "mechanic". Drove the car for about 10 or 15 minutes. Drove fine. Seemed a little underpowered but I'm not familiar with this vehicle as I'm working on it for a customer. Made it back home and let it idle for maybe five or 10 more minutes and attempted to give it gas and it's sputtered and acted like it was starving for fuel. The temperature gauge is pegged all the way to zero and does not move. Would that cause the issue or should I start my tests elsewhere? I will be testing the resistance on the coolant temperature sensor and the fuel pressure tomorrow. Just wondering if anybody has a smoking gun on this issue as it seems to run okay at the beginning. Thanks

[Spider951]

There are more than a couple of possibilities. Just for starters, yes it needs a functioning coolant temperature sensor (located on the coolant "T") to send coolant temp info to the ECU (which then adjusts air flow sensor as needed).
There are also two sensors located on the head, one located between spark plugs 1 and 2 (coolant thermostatic switch, I think it's called), and the other between plugs 2 and 3 (heat gauge sending unit) - make sure electrical connectors to both sensors are good (those wires tend to get disconnected or otherwise damaged). That may resolve the issue with the gauge needle on the instrument panel.
Since fuel seems to be a question, what gasoline is in the tank? - my '81 FI runs waaaaay better with non-ethanol gasoline - even the 87 octane non-ethanol stuff is far superior to the best 93 octane ethanol-containing gas money can buy in my part of the country. These are 40+ year old cars, designed years before ethanol came on the scene. Others will have a different tale, and tuning can be achieved (apparently) to run on ethanol - I'm just saying what I have found that works best for my car. Besides, it's an easy test to run, if non-ethanol gas is available.
Other thoughts: age, condition of fuel injectors? spark plugs - running rich? lean? [I also had to lean my Air Flow Meter a bit to get best results.] Take a look a Brad Artigue's write up on L-jet FI for these cars - I think it is available on Mirafiori.com website (?).

[davidbruce]

When you changed the tank did you back flush the fuel lines. From the factory the fuel filter is after the electric fuel pump on the FI cars. When I purchased my '80 it was a non runner. The culprit was a fuel pump full of rust. I have since installed a clear coarse mesh filter in front of my new pump. Whatever was in the tank to cause you to change it could new be fouling the pump and or injectors. As well as a pressure test I would do a volume test as well. Also as 951 pointed out check those 3 senders and their wiring.

[Truthhurts21]

Thank you for the replies. I did get the coolant temperature sensor tested and it is working correctly turning the fan on at the proper temperature according to my infrared thermometer. The two sensors on the head in between the spark plugs were swapped so that's why the gauge was not working. So when I switched them it started working correctly. Yes I did read some forum post about the fuel pump possibly having crud inside of it. The tank honestly was not that bad inside but the customer wanted it replaced on advice from someone else. It's just strange that it will run fine the first 15-20 minutes I get all the way back and park it and let it idle for another 5 minutes or so and then go to give it gas and it sputters and will not gain RPM quickly. Seems heat is causing something to fail but my next test will be the fuel pressure as I have a gauge and all Tee in an adapter for it and drive it. We shall see. Thanks again.