Let me see if I can explain this… I had a hesitation issue and I found a bad injector clip to be culprit. Prior to fixing this, my car developed a power loss at 4,000. When I replaced my secondary injector clips the power loss was still there so I did a fuel pressure test. My results were 30 psi between the engine and the filter warmed up. By the FSM that’s normal. So I taped the fuel pressure gauge to the windshield and notice my fuel pressure drops to 20 psi at 4,000 rpm and when turned off, the pressure drops to 15 psi immediately. What has me lost is that my 4,000 power loss went away, but it returned a few hours later after taking off the fuel pressure gauge. So should my fuel pressure increase or stay the same when driving? And could a Bad fpr cause this? BTW this is a stock N/A 86 rx7

 

probe the fuel pump power wire and monitor the voltage while driving.

and dumb question but...

have you changed the fuel filter?

fuel pressure follows your foot, when it's buried pressure increases, when decelerating with no throttle pressure decreases.

 

Yes I changed my filter. My old pump rusted out in the tank because the car sat for 10 years. I got another pump of a parts car. I told the guys at rotary performance and they said it's probably not the filter. How do you probe the pump and monitor voltage? Thanks!

 

There's a four wire plug which connects to the fuel pump pigtail and on one side of the connector is a Blue/Green wire (delivers voltage to the connector/pump pigtail) that mates with the wire from the pigtail which I think is Black. You check for the voltage on the pigtail side of the connector.

 

it's black/white between the pigtail and the top of the pump, changes to a blue color wire once inside the tank.

 

It read 11. something volts. I"m going to drop the fuel tank. I put my hand in there and grabbed a bunch of black crystal things, and the pump has surface rust too.

 

might want to consider a replacement tank.

 

Cleaned out the tank and it looks much better. I retested the fuel pressure and it was normal at idle. One thing I notice is that when I floor it the pressure drops to 10 psi or 20 psi and that's when the hesitation occurs. When the pressure spikes up to 30 psi the hesitation stops. Bad fuel pump maybe? I thought when a fuel pump goes out it dies all at once. BTW can I pick up a pump from O'Reilly. I know if you get one that flows too much it can make the car run rich.

 

sounds like a restriction but could be a bad pump and is putting out low volume. the pressure should be about 32psi at idle and about 40psi under a load.

 

Can this rule out bad injectors? The guys at rotary performance said I might need to get them cleaned. He said maybe the injectors are not flowing and higher rpm. Thanks

 

I would think common sense would dictate that if they weren't flowing the resulting fuel pressure would increase when tested and not decrease. If you tested for fuel pressure w/key to on and the fuel check connector jumpered the pressure should be almost 30 psi and when the vacuum hose to the FPR is disconnected it should jump at least 5 psi or so.

And you have a Fuel Pump Relay & Resistor which should increase your pressure under load and it might not be doing its' job.

 

If you just replaced the fuel pump. Make sure there is a very good seal from the new fuel pump to the metal fuel line in the fuel pump assembly. I just recently replaced mine and observed a bad or loose connection. There must be constant pressure for the system to work right.

 

I was thinking the same thing too. I keep having people telling me it might be bad injectors. With the FPR off it went to 30 psi to 40 psi.

 

You might want to test the voltage at the fuel pump and compare the reading you get when cruising at 2k to 3k rpm and compare it the voltage reading when you mash the pedal and try to launch the rpm upwards to see if it drops.

 

Given that your anomaly is at 4,000 rpms and the car sat for a long time I would go thru Aaron Cake's ground cleaning procedure. The cars are notorious for stumbling at 3800 rpm because of bad grounds. This is a good thing to do regardless and it cost almost nothing to do since all you need is some sandpaper and dielectric grease with stainless steel replacement fasteners a bonus.

Here's the link...

Good luck!

 

i was assuming he knew that is what i meant, but i suppose not everyone can read my mind. check the voltage during the problem area and i bet it's dropping voltage due to a faulty 2 step relay. the crappy tank is still an issue that needs to be resolved as well though.

 

I will try this some time this week along with a used pump and filter. I went to Rotary performance and showed them the pictures I took of the stuff in my fuel tank and the condition of my pump. They told me i should highly replace my fuel filter. They said something about how the new gas formula probably cleaned out all the 10 year old crust in my tank. So i guess possibly after 8 months of driving broke down the sediment in my tank clogged that filter.

 

A little update. It was the fuel pump. I took off the sock and found crud in the pump. I also replaced the fuel filter and fuel hoses connecting to the filter. She runs really strong and smooth. Thanks for the help guys.

 

you may want to consider yanking the tank out and having it treated or replaced. the issue may popup again in the near future otherwise.

 

I know its a bit late, but do S4 n/a cars have the fuel pump relay that goes from 9v to 12v like the other FC's do?

 

NA no, turbo yes.

 

series 5 does, series 4 doesn't. just in case a S5 owner trips across this question.

 

Yeah I added that on my parts list. I'll have to research about having my tank treated or find a good parts car with a clean tank.Thanks!