I ran a fuel pressure test on my 1988 F-350 5.8L 4wd today - I'd like to share the results and get some advice/recommendations?

When I turned the key on the 1st time withou starting the engine I received 0 psi. I repeated this three more consecutive times and still registered 0 psi.

Next I started the engine and the pressure slowly but steadily rose up until it steadied at 34 psi. It took about 15 seconds for the pressure to rise up to 34 psi.

I disconnected the vacuum line to the FPR and the pressure rose to 42 psi.

After the engine ran for several minutes at 34 psi fuel pressure I shut it off and the fuel pressure dropped at this rate:
After 20 seconds dropped to 20 psi;
After 1 minute dropped to 12 psi;
After 2 minutes dropped to 8 psi;
After 3 minutes dropped to 7 psi;
After 4 minutes dropped to 7 psi.

Then I turned the key on 6 times without starting the engine in order to monitor key on - engine off fuel pressure.
1st key on-engine off rose to 14 psi;
2nd key on-engine off rose to 14 psi;
3rd key on-engine off rose to 17 psi;
4th key on-engine off rose to 20 psi;
5th key on-engine off rose to 23 psi;
6th key on-engine off rose to 25 psi.

I've read the Codes with an OBD I Innova 3145 and the only code thrown is Code 33 in CM and during the KOER mode.

The primary troubling symptom I experience is that the truck tends to stall when stopping at stop lights (idle), or after start up when shifting from Park into Reverse or Drive. I have to step on the gas peddle a bit and maintain rpms to prevent stalling at idle.

The truck has two (2) low pressure assist fuel pumps, one in each of the two separate fuel tanks, and one high pressure fuel pump mounted to the left frame rail. I'm thinking I should replace the high pressure fuel pump and add a can or two of fuel injector cleaner to the fuel tanks. Truck has 120,000 miles on it. None of the fuel pumps have ever been replaced. Brand new fuel filter downstream of the high pressure fuel pump was installed one week prior to running this fuel pressure test.

How do the Forum member interpret these fuel pressure test data? Soliciting recommendations? Thanks,

 

The FP test results appear to be normal... but I don't know the specifics for your truck and I don't have my service manual handy so I can't say that w/ 100% certainty. But as far as the decline and increase goes, it doesn't look bad.

Don't go replacing the hi pressure pump just yet. Mileage means next to nothing as long as the pumps have been operating normally.

What I would suggest is to test the pressure with the truck in drive and your foot on the brake. Obviously this requires the help of a friend, but that will tell you whether or not fuel pressure is the problem. Just make sure the engine is up to temp first.

I had a stalling/idling problem w/ my 91 F150 and it turned out to be a bad TPS. If the FP test comes back normal then try testing the TPS along with some of the other sensors and see what you come up with.

 

Nope I'm afraid I'll have to disagree with my fellow poster above, them are **** pour FP test results. Should build higher value quicker.

Same results both tanks?

Fuel pressure should hold within 5psi of the specification for 1 minute after shutting off the key.

You should get 35 to 45psi engine off, 35-45psi engine running vac line to FPR connected and 5 to 10lbs increase vac line disconnected.

Low pressure tank pump test= 6oz minimum in 5 seconds.

Need to know if it effects both tanks but so far sounds like a FPR problem, especially if it does act the same with both tanks.

Pinch the fuel return line downstream of the FPR, test the fuel pressure again, if the pressure rises substantially, replace the FPR.

If no change the pumps or supply line would be in question, filter to but you replaced that. Might pull the filter, make sure its not obstructed by rust from one or both tanks, say if the in tank screen fell off or degraded. Doubtful but.....

 

I thought the fuel pressure should have built up with the key on-engine off, during the 2-3 seconds where I hear the fuel pump run before it turns off. But I got zero psi - no pressure at all after about 4 attempts. So then I started the engine and collected the fuel rail pressures posts above.

I'll run the fuel pressure test again tonight with the tank selector on the other fuel tank and see if there are any changes. Then I'll post that data for review and comments. Thanks.

 

Ran another fuel pressure test with the tank selector switched to the other fuel tank. Basically the same results.

When I start the engine it takes about 20 seconds to come up to full stable operating pressure of 34 psi. I put the transmission in gear and stepped on the brakes and throttle and the fuel pressure rose to 40 psi.

When I turn the engine off, fuel rail pressure drops at this rate:
10 seconds > 28psi
20 seconds > 22 psi
30 seconds > 19 psi
40 seconds > 17 psi
50 seconds > 16 psi
60 seconds > 14 psi
90 seconds > 12 psi
2 minutes > > 11 psi
3 minutes > > 9 psi
4 minutes > > 9 psi
5 minutes > > 8 psi
7 minutes > > 8 psi
10 minutes > >7 1/2 psi
15 minutes > >4 psi

Then I turned the key on four times within 60 seconds to see how much fuel pressure was generated during the 2-3 second fuel pump run time.
1st Key On > 5 psi
2nd Key On >12 psi
3rd Key On > 17 psi
4th Key On > 20 psi

Then I quickly turned the Key On four additional repetitive times and checked the fuel pressure. After this 8th Key On cycle the pressure read 40 psi and then starts dropping off similar to the above posted rate.

I forgot to pinch the fuel return line downstream of the FPR. Duh...

From what I've read it seems like I have a weak high pressure fuel pump? Others have fuel pressure jump up to 40 psi with just the 2-3 second Key On Engine Off fuel pump cycle. I only get 5 psi. Even when I start my engine and the fuel pump is running continuously, it takes 20 seconds to build fuel rail pressure up to 34 psi. Others have posted that they've never experienced a bad FPR producing too little fuel rail pressure. When they experience FPR failure they see elevated fuel rail pressure. When I pull the vacuum off the FPR, fuel rail pressure jumps from 34 to 42 psi - as expected.

Why the rapid rate of fuel pressure loss when I turn the engine off? Leaking fuel injectors? Or fuel leaking backwards through the FPR?

The engine runs reasonably well, so the fuel pumps seem to keep the engine supplied with enough fuel for the most part, but my pressures seem skewed compared to what others have posted.

I took the IAC off tonight, cleaned it out and reassembled it. The pintle was moving freely. Whether or not the solenoid is functioning properly I don't know for sure. With the engine running I unplugged the solenoid connector and the engine slowed some but idled just fine. I could tell some difference, but the engine idle speed seems to drift around more than necessary with the IAC connector plugged in. I sprayed cleaned the throttle body with carburator/throttle body cleaner.

Then I tested for trouble codes again. This time I received the same KOER code 33, but I also received a new KOER Code 12. My Innova book says a code 12 means "RPM at idle out of range/high". I haven't researched that Code 12 yet.

Is there any opinion or concensus on the fuel pressure test data? Thanks.

 

Sounds like a bad fuel pressure gauge to me.
Number one an engine will not start at zero PSI fuel pressure while turning the key off and on.
To make that a little clearer it will not start or run below 27-28 PSI.
Turning the key off and on only needs to get you to about 30 PSI to start the engine.
Never use key on and off to test anything.
The bleed down rate in the last post is OK. The only thing here is you have to make sure the fuel lines do not drain overnight or will take a long time to start the engine in the morning. As long as the fuel lines are full and at zero pressure the engine should start as you turn on the key (jumps to 30 psi) and crank the engine.

Before you change anything mount the fuel pressure gauge someplace where you can see it (with tape) and take it for a drive. If it acts up and you have more than 32 PSI at idle and 40 PSI at WOT the fuel system is OK.

 

Fuel pressure should build to about 35psi(no start) with no more than two cycles of the key and instantly when the engine is started, and it should immediately jump to 40-45psi when the throttle is blipped from idle or vacuum is removed fro the FPR.

If it hasn't been done already change the fuel filter and have a look at the fuel switching valve, if this unit is leaking internally fuel would simply be pumped back to the tank and bypass the engine which will make it hard for the pumps to build the correct pressure at the fuel rail.

 

Good points. I don't know why it's taking the pressure gauge so long to pressure up with the engine running. It gets to 20+psi in ~5 seconds and then takes another 10-15 to build to 34. This fuel pressure gauge and test kit was purchased about 2 yrs ago and has only been used about 4 times. It does appear the gauge is slow to respond because the engine fires up pronto - meaning the fuel pressure is actually high enough to fire through the injectors. I guess I'm not going to worry too much about that lag in pressure build because the engine does start quickly, even when it's not been started for days. So the fuel rail and fuel lines are evidently remaining primed and full of gas.

Thanks much for the feedback on the fuel pressure test. Glad to hear that my fuel pressure bleed down rate is acceptable. Based on your comments, the fuel pumps and FPR seem unlikely to be the cause of the engine stalls at stops.

I added a couple cans of fuel injector cleaner to the foward tank into about 10 gallons of fresh gas. It can't hurt anything and may help improve the injector spray patterns. I'll install the fuel pressure gauge where I can monitor fuel pressure while driving and see how it goes.

I'm now turning my suspicions towards the IAC solenoid. I'll take some ohm readings this evening if I can locate the pertinent specifications. The solenoid may not be responding quickly enough to open the air passage way as the engine returns to idle - prior to the engine stalling.

 

Thanks for commenting. I did install a new high pressure fuel filter on the truck just shortly before I ran the fuel pressure tests. The vehicle was still stalling at stops after I replaced the fuel filter.

 

Here is some information on the IAC valve.
Ford Fuel Injection Idle Air Bypass (IAB)

/

 

You probly have gummed up fuel pressure gauge. I had that happen to me once and gave kinda the same readings you have. The old gas inside the guage gums up the gauge's small passages hence the wierd readings. I since then taken carb cleaner and washed it out after use.

And yes that cost some $$$ running after a wild goose chase!!! At least I found the problem.....nothing was ever wrong with the fuel system at that "learning" experiance.... dang pluged cat(on my 85 Plymouth about 12 years ago)!

 

I'm going to jump in here with my issue - no offense intended - but I have an unusual FPR reading and it seems like the experts are on this thread.

I have almost the same vehicle with a 5.8l engine. I failed inspection for high hydrocarbons. I ran a search on this forum for high hydrocarbons and have been checking out what was suggested - codes (all good), IAC (cleaned) and then the FPR readings. I read 30-PSI at idle, 40PSI with the vacuum line removed. After tuning the engine off the pressure steadily increased to 40PSI within a minute. Are these readings a sign of a faulty FPR?

Thanks

 

Sounds a little low but it should have the opposite effect on your high hydrocarbons. They should be lower (lean fuel mix) not higher.

You might want to try a 2 cans of heat in the fuel tank that you will be running on when the test is done and make sure the engine is well warmed up with a 195* T-stat.
You may want to test your ECT sensor also.

 

Thanks, you don't see anything weird with the rising to 40PSI after the engine is off?

 

Nothing wrong with that, as stated the fuel system should hold ~40psi with vacuum removed from the FPR at idle so that means it should also hold 40psi with the engine off, that shows your injectors arn't leaking and the check valves in the delivery module are also holding.