Bad Hesitation When Heat Soaked

[My5ABaby]

'86 N/A
Mods: Racing beat header double wrapped, RB presilencer wrapped, emissions removed.

The car runs and pulls great until it gets very heat soaked. At that point it starts cutting out. At low throttle it kind of bucks, at high throttle it almost starts coasting and gets quiet as if nothing is working at all (although the rpms don't change at all).

Example 1: Friday I was on the interstate going "the speed limit" with no hesitation issues for about an hour (was pretty hot out). I got to a large mountain and it started hesitating to the point I had to slow down. After the downside of the mountain it was better. About 30 minutes later it started hesitating. In 5th gear I couldn't give it more than about 30% throttle and go over around 70.

Example 2: I was at Talladega on Saturday and had two 20 minute runs with about 35 minutes between them with no issues. On my 3rd run it started after about 5 laps. It initially began as cutting out in turns (both direction), and eventually it was cutting out to the point I had to pull off. At this point I had about 2/5ths of a tank of gas.

Example 3: On the way back from Deals Gap it started doing this and go to the point I couldn't even rev it past ~4k in neutral.

I would normally think 3800 rpm hesitation but revving in neutral shouldn't even engage the secondaries. Someone suggested maybe the fuel pump was getting too hot on a lower tank of gas but it did it on Saturday even with a full tank so I'm thinking that would rule it out. I also thought maybe the pump sock was dirty (normally making it cut out turning in one direction), but it did it in both directions and on a full tank of gas.

The car was also mostly fine on the way home this weekend as the temperatures were much cooler. If I tried to punch it the car would buck a little, but anything under about 70% throttle was fine.

After the Gap but before Talladega I reground the entire engine bay. Quite a while ago I added a boost sensor ground (to fix the 3800 rpm hesitation).

Ideas?

[Rx-7fetish]

Which wire on your boost sensor did you ground cause on 86's its not the black, and its different from any other year. I have an 86 too, ill check the exact color and get back to you tonight, ive done the extra ground too.When was the last time you changed your fuel filter?

[TitaniumTT]

Have you checked the voltage @ the pump when it starts doing this? it could be the pump/wires are getting warm and it's not getting the voltage/amps that it needs.

[djmtsu]

Same thing my RX-8 was doing when the fuel pump took a dump.

[TitaniumTT]

I seem to notice an awefully large number of fuel pumps puking lately. My Bosch 044 puked on me.

[My5ABaby]

Originally Posted by TitaniumTT

Have you checked the voltage @ the pump when it starts doing this? it could be the pump/wires are getting warm and it's not getting the voltage/amps that it needs.

No, I haven't had a chance to do much diagnosing. Wasn't really sure where to even start.

If that is the issue how would I fix that? I have all of the rear heat shields in place.

Originally Posted by Rx-7fetish

Which wire on your boost sensor did you ground cause on 86's its not the black, and its different from any other year. I have an 86 too, ill check the exact color and get back to you tonight, ive done the extra ground too.When was the last time you changed your fuel filter?

I think it was black/brown. It got rid of the 3800 hesitation when I did it a while back.

Originally Posted by djmtsu

Same thing my RX-8 was doing when the fuel pump took a dump.

That's a possibility. That's one thing Billy (respeed) thought too. It's a stock pump with unknown mileage.

[TitaniumTT]

If it was me, and I'm a total n00b when it comes to diagnosing fuel pump problems I would first Get a voltage reading at the battery, they start the car and get a second voltage reading.

Turn the car off and probe the back of the connector of the fuel pump..... I think you're looking for a B and a L wire. If the multimeter has a hold or peak function, hit that and see what it jumps too.

Turn the hold function off and start the car and see what the voltages are. You should see 12ish volts at the bat, 11+ with the pump running and no start. High 13's with the car running and over 12 with the car running and the fuel pump pumping.

Next it's time for an Amp test. The connector needs to be unplugged but jump the gnd with a length of wire. Insert the multimeter between the hot wire and record the Amps, should be like 10-12A on the highside.

I'm betting that the volts are low or the Amps are high. If the volts are low, re-wire the pump and see what happens. If the colts are good and the Amps are high, than the pump is going bad.

The heatshields... I believe.... have very little to do with it. It's more a function of the pump running hard for hours on end and getting hotter and hotter and hotter and more and more resistance and blah blah blah things just get worse over time.

it could also be a clogged fuel filter or a bad fuel sock. Both of those things will lead to more physical resistance which causes the fuel pump to work harder and get hotter and the same thing can happen.

The key is to determine if it's a physical problem or an electrical problem. What's the inside of the tank look like?

[My5ABaby]

Originally Posted by TitaniumTT

If it was me, and I'm a total n00b when it comes to diagnosing fuel pump problems I would first Get a voltage reading at the battery, they start the car and get a second voltage reading.

Turn the car off and probe the back of the connector of the fuel pump..... I think you're looking for a B and a L wire. If the multimeter has a hold or peak function, hit that and see what it jumps too.

Turn the hold function off and start the car and see what the voltages are. You should see 12ish volts at the bat, 11+ with the pump running and no start. High 13's with the car running and over 12 with the car running and the fuel pump pumping.

Next it's time for an Amp test. The connector needs to be unplugged but jump the gnd with a length of wire. Insert the multimeter between the hot wire and record the Amps, should be like 10-12A on the highside.

I'm betting that the volts are low or the Amps are high. If the volts are low, re-wire the pump and see what happens. If the colts are good and the Amps are high, than the pump is going bad.

The heatshields... I believe.... have very little to do with it. It's more a function of the pump running hard for hours on end and getting hotter and hotter and hotter and more and more resistance and blah blah blah things just get worse over time.

it could also be a clogged fuel filter or a bad fuel sock. Both of those things will lead to more physical resistance which causes the fuel pump to work harder and get hotter and the same thing can happen.

The key is to determine if it's a physical problem or an electrical problem. What's the inside of the tank look like?

I'll check the voltage/amperage as soon as I can find my DMM. Not quite sure where it ran off to... I'll also look in the tank. I doubt it's in good shape.

[FerociousP]

Originally Posted by My5ABaby

I'll check the voltage/amperage as soon as I can find my DMM. Not quite sure where it ran off to... I'll also look in the tank. I doubt it's in good shape.

You may have left your multimeter IN the tank? You've got more than just fuel pump problems my friend.

[My5ABaby]

Originally Posted by FerociousP

You may have left your multimeter IN the tank? You've got more than just fuel pump problems my friend.

I guess I should have separated those thoughts a bit better.

[TitaniumTT]

Originally Posted by FerociousP

You may have left your multimeter IN the tank? You've got more than just fuel pump problems my friend.

nice

Originally Posted by My5ABaby

I guess I should have separated those thoughts a bit better.

Maybe, but I understood it