OK, the loaner fuel pressure gauge from AutoZone is missing the pin inside that releases the valve to get pressure!!!! That explains ZERO Pressure

I'm starting to dig into tuning the 89 after the rebuild 6 months ago and want to make sure to start with fuel pressure is right.

Do I need to check the pressure at different RPM levels? just at idle?

Any Do's and Don't to make sure I get proper diagnostics?

 

I believe the fuel pressure for an L98 should be 43 pounds at idle. I think that this reading is taken with the vacuum line pulled, but other will correct me if I am mistaken. I do not believe that you need to take it at other RPMs. Now if you had problems with stumbling at high RPMs, look at the pump or the filter.

 

Fuel Rail Pressurization:

When you first turn the key to the “on” position, the fuel pump will run for 2 seconds pressurizing the fuel rails. There is a Shraeder valve on the passenger side fuel rail near the rear of the engine and if you measure the pressure there after the pump runs, you should see between 40-42 pounds of pressure. The reading will go to 38-40 pounds nominal once the engine is running.

I don't recall what the FSM says for sure. But i is close to this if not exactly.

I don't know why you would need to check it at different RPM's. But hey why not.

 

OK, I think I'm losing it.

I just hooked the gauge up to the vette and I get ZERO pressure!!!!

I've tried to hook it up once directly to the gauge and once used the 45 degree fitting and both time no reading at all!!!!! put on hand tight but after no reading I did tighten with a wrench a bit tighter.

I've pressed the release valve and get nothing either. Here is a pick of the hookup.



Can anyone offer some help, can't believe it is this hard!!!!! Some how given the car runs fairly well, I don't this this is a valid reading!

 

Dolfan
Make sure the Schrader valve is being depressed.
If the valve is not depressed the reading will be 0
If you have the stock fuel pressure diaphram it will be in low 40 psi range

 

OK, replacement of the gauge and now I get a rock steady 38 at idle and at higher RPM. Now to look at timing and other things to try to smooth thing out.

 

fuel usage at idle and high revs is small with NO LOAD, much greater when engine is under load...not common, but dirty filter/worn pump/etc may produce satisfactory read at parked/'no load' but reduced pressure read under actual driving conditions...tie gauge to windshield wiper,etc, and have helper watch gauge closely for press DROP while you accelerate or go uphill, (helper 'should' report pressure RISE when driver depresses throttle pedal from idle, this is normal/correct)

 

nope

 

when it comes to injectors I'm a dumby. There are some simple things you can look for after you rule out fuel pump and FPR like fuel around your plugs and such or try to look for symptoms that point to a leaky injector. But that is not always 100% accurate, it just gives an idea.

Jon at Ficinjectors is the expert around here on thos matters. I'm sure he will be more than happy to explain it for you.

He has a sticky here in the tech section with some great info.

http://forums.corvetteforum.com/c4-t...questions.html

Here is his web page

http://fuelinjectorconnection.com/shop/

There are also some pretty knowledgeable folks around so we will bump this up in hopes one of them are available. floridamale is knowledgable and so is Frizlefrak along with many others

These fuel delivery issues on these things can be a PITA.

 

I found this in the archived tech tips. Good info!

Here are the sure fail tests for the fuel system to save you time and money.. Take a fuel pressure gauge attach it to the schraeder valve.. turn the key on and off a few time to build up the pressure in the system... If you are having starting trouble the pressure will probably drop as soon as it hits 38 or 40 and the pump turns off it will probably leak back to zero within a few minutes. Ok now right after you prime the system pinch off the return line near the tank if the pressure drop stops it is a bad fuel pressure regulator.... Next pinch off the feed line if the pressure holds then the leak is before the point where you pinched the line IE fuel pump check valve..hole in the metal line on the sending unit caused by it rubbing on the inside of the tank you should visibly be able to see this if you pull the pump and the sending unit...(this was my problem) or the pulse dampener which is the piece between the pump and metal line on the sending unit.. the diphragm tears.. and last if you pinch both the feed and return line near the tank and the leak down of the pressure continues then a fuel injector is your problem.. This is the GM test.. saves time and money..

 

Squeezing both hoses at the tank COULD indicate a pressure loss anywhere forward of that point; fuel line, filter, regulator, etc. So doing it there isn't definitive.

On the passenger side of the engine, where the lines begin to move UP toward the intake was where I squeezed next, eliminating every thing rearward.

i finally ended up removing the fuel rails with the injectors still attached, energized the pump and watched for any injector leak - none found.

My pressure loss turned out to be on the pump itself. Easy to fix. Then I spent an hour or so timing the pressure loss rate.

Moving forward in your checking will help pin point a leak quicking, if there IS one. That's how I attacked my 86 years ago.

Jake

West Point ROCKS! Nation's TOP COLLEGE per Forbes Magazine!! Graduation Day Parade 20 May 2010!!!

 

Just unscrew it but be careful not to spill any fuel when you take it off. Use a rag as a glove to absorb any fuel that may drip.

Do the check with the car off and then with the car on. I don't recall the exact amount of time to wait after each step to see if it bleeds down,. probably 2 to 5 minutes will give you an idea if it is bleeding down too fast.

Here is a basic video that will show you the basic steps. Youtube is full of them. get your book out and watch these videos and play with it until your comfortable. They also have tests on the FPR and the fuel filter and pump.

Once you do these steps come back if you have more questions.

http://www.youtube.com/watch?v=ERZe6Vrl7to

 

IF IF IF the gauge is good, 'sumptins' wrong...driving with press that low may melt a piston, do not.

 

That's low. Your loosing pressure somewhere. Do the other tests to rule out the FPR and the Fuel pump. Could be a filter also but I doubt it.

Is there any smoke or smell of fuel?

 

HeY Josh,

i'm inclined to think your pump or FPR is acting up. But again I'm not an expert.

I correct myself in that it could be a bad filter or dirty sock as well.

Do those other tests. Also with the engine running take the FPR line off and check to see if your pressure goes up as it should.

Also is their any smoke or smell of fuel?

 

i'm not sure how long it should hold exactly. It will go down some but it should hold a few minutes. i'll have to look at my manual tonight and see what it says, if it does mention it.

Sounds like your FPR is OK and if the PO changed the injectors they should be ok but that is not absolute. You get no smoke or smell of fuel, that's positive.

get someone to help you pinching off those lines, or get a long hose on the gage.

what exactly are the issues you are having? without finalizing the checks by pinching off the lines it appears everything is checking out so far.

When you replaced the pump did you replace the sock? I have read those will decrease fuel pressure as well, though I have no first hand experience with it.

Edit: I read in the other thread what your issues are. Fouling plugs I believe? The PO put in 24lb injectors and I believe the L98 takes 22 lb.

I would give Jon at ficinjectors a call and see what he says about that. Could be your getting too much fuel and fouling your plugs. But I would think you would smoke a little bit. But Jon is the man to ask.

 

I'ld say it is the 24Lb injectors myself. The L98 uses a 22Lb. also I did read in my FSM that if the pressure drops below 26, I think, that you have a serious issue and not to run the engine. 34 could indicate a lot of things from filter to the sock on the fuel pump. Also just because the pump primes up good with engine off doesn't mean it's pumping good at a load.

keep your eye on it and let us know if anything changes.

 

I am in the middle of THE fuel pump night mare now.I do think after reading all this your problem is the 24 pound injectors.I worked with fic and he got me set up with 22lb Bosch111's they work killer the car never ran better .I am having a hard start with a long cranking time as my pump preasure goes to 0 instanly.I have traced it to the pump as it goes to 0 right after I shut off the key(gauge hooked directly to the pump).Good luck and talk to Jon at fic he will hook you up!
UPDATE: New fuel pump and holds pressure and starts with the turn of the key!!

 

Hey the best of luck on that, the spark plugs could be the issue.I have the best luck with ac's in gm stuff,just my pref. I love this sight as all the experience we get to enjoy from all the vette dudes (and gals) have fun!! I am sure happy now as my issue is done.But of coarse on to next one!