Fuel pressure issues

[98BABCobra]

So my 98 cobra started taking longer thann usual to start up. I noticed if I cycle the key a few times to prime the fuel pump then it starts right up, so that tells me it's not an air or spark issue. I initially thought my fuel pump was going bad, so I replaced it with a brand new one, but the problem persisted. I rented a fuel pressure testing kit that connects to the schraeder valve on the fuel rail and tested the pressure. My pressure is within specs at idle, and when I take the vacuum line off the pressure regulator, the pressure rises like it's supposed to and is still within specs. When I turn the engine off to check the holding pressure, its only supposed to drop 5 psi after five minutes. Well my pressure dropped all the way to 0 within 2 minutes.

I suspect my pressure regulator is going bad as I have checked for any other fuel leaks in the lines, and last weekend I pulled my fuel rail and inspected the injectors and installed all new injector o-rings (all 16). Does this sound like i need a new fuel pressure regulator? Thanks guys and sorry for the long post.

 

[mwolson]

Could also be a leaky fuel injector or a bad check valve in the fuel pump. I suspect it isn't the injectors since you are not seeing a rich DTC. I think the check valve is more likely, since I can't figure out how the FPR could cause the leak down, since it is working ok at idle. I'd pull the plugs and look for a rich cylinder first since that's easy.

 

[98BABCobra]

Could also be a leaky fuel injector or a bad check valve in the fuel pump. I suspect it isn't the injectors since you are not seeing a rich DTC. I think the check valve is more likely, since I can't figure out how the FPR could cause the leak down, since it is working ok at idle. I'd pull the plugs and look for a rich cylinder first since that's easy.

I don't think it could be a bad check valve in the pump as that was my first suspicion when this problem started, so I put in a brand new pump less than a month ago.

As for pulling the plugs to look for a rich cylinder, would I just be reading the plugs looking for one that is fouled up? Thanks

 

[mwolson]

Well that eliminates the check valve.

There are a ton of pictures on the internet that show rich fouled plugs. That's what I'd look at next in your situation. If you can read DTCs, I'd do that too. You can have a DTC without a CEL.

 

[98BABCobra]

Ok I'll pull out my Xcal3 and check for codes. I think my Haynes repair manual also has a chart inside the back cover with a guide to reading plugs. Thanks again for the advice.

 

[98BABCobra]

Well last night I checked for any DTC's and there weren't any. Hopefully tonight or by the end of the week I will have time to pull the plugs and hopefully be able to tell whether or not I have a leaky injector/injectors.

 

[mwolson]

I would have expected a rich condition DTC if you had a leaky injector. This is perplexing.

 

[98BABCobra]

It is very bothersome. The car runs perfectly once it has started up. It's just this issue of low pressure upon start-up. I don't really know what the problem could be, other than the FPR or injectors. The fact that the fuel pressure is good at idle both with and without the vacuum line connected to the FPR makes me think that is not the problem.

 

[mwolson]

I agree. I don't think it's the FPR. Unless it has a really funky failure mode. Is it possible for the fuel to get past the diaphram and into the vacuum side? You might try using a vacuum tester to see if the FPR holds vacuum.

And you don't have any leaks in the fuel line. Very perplexing. The fuel has to be going somewhere.

 

[98BABCobra]

I need to trace my fuel lines just to double check, but I am pretty sure there are no leaks along any of the lines.

According to my Haynes repair manual, if the pressure leaks down after turning the engine off, the probably culprits are a bad FPR, leaking injectors, or another fuel leak somewhere. Lol, I know that doesn't narrow it down any further than where I am at now.

 

[98BABCobra]

Well I finally got around to pulling all of my spark plugs this morning and inspecting them. They all looked perfectly normal with no signs of running rich. So I'm thinking its gotta either be the FPR or a leak in the fuel line.

 

[mwolson]

Have you checked if the FPR will hold vacuum with a vacuum tester?

 

[98BABCobra]

No I haven't done that. Is that something I should be able to rent from a parts store?

 

[mwolson]

You might be able to get a loaner from a parts store. Mine is like this one, but also has a brake bleeding cup: http://www.carid.com/actron-hand-tools/actron-testers-and-tools-15891020.html

 

[98BABCobra]

Cool. I'll look into it