TPI fuel pressure issue



TPI fuel pressure issue

Postby grumpyvette » May 23rd, 2014, 2:17 pm

Timothym71 wrote:I have a 1989 350 TPI engine in a street rod I built. I am having trouble with fuel pressure and did the diagnostics, which pointed to a faulty fuel pressure regulator. I replaced the fuel pressure regulator with an adjustable one from South Bay. I now have good fuel pressure while the car is running (43 with vacuum off, 37 on), but as soon as I turn off the car the fuel pressure immediately goes to zero. Also, on WOT, the pressure drops to about 5-10 pounds and cuts out. Again I did diagnostics... when I pinch the return line the pressure shoots up to 80 pounds and will hold until I release it. I can't wrap my brain around what could be causing this other than the fuel pressure regulator. Any help would be greatly appreciated.

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from your description, new or not, it sure sounds like your fuel pressure regulator diaphragm is leaking,or the regulators defective, reading thru the links should help. the fuel rail pressure should slowly drop with the engine off over more than 10 minutes time at the fastest,, having a fuel pressure regulator fail is a very common problem

http://members.shaw.ca/corvette86/FuelS ... gnosis.pdf

viewtopic.php?f=55&t=1241

http://chevythunder.com/

viewtopic.php?f=55&t=635

viewtopic.php?f=32&t=10688&p=46303#p46303

viewtopic.php?f=55&t=1378
IF YOU CAN,T SMOKE THE TIRES AT WILL,FROM A 60 MPH ROLLING START YOUR ENGINE NEEDS MORE WORK!!"!
IF YOU CAN , YOU NEED BETTER TIRES AND YOUR SUSPENSION NEEDS MORE WORK!!
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Re: TPI fuel pressure issue

Postby philly » May 23rd, 2014, 5:18 pm

same issue on the callaway and a new diaphragm cured it... we used the bbk one not the south bay
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Re: TPI fuel pressure issue

Postby bytor » May 24th, 2014, 6:00 am

I just checked out the TPI fuel system on my 87 vette the other day. I replaced the fuel pump because it was making a horrible noise. Turns out I had a kink in my return line right where the rubber hose connect to the sender. I replaced the pump anyway.

I did a leak down test. Got 44psi with key on and no start. It held but dropped to 38psi after 2hours. Is that ok?

Started the car and got a steady 38psi at idle. Removed the vac hose on the regulator and got 48psi so it seems my regulator is functioning normal. I checked the regulator vacuum line for fuel and didn't see any. However, I noticed a faint 'smell' of gas when I sniffed the vacuum hose that connects to the regulator. Also ohmed out all ma injectors. They all came in at 16.5 ohms. Question, on the 1987 vette, who's the manufacture of the injectors? Rochester? My understanding is the Multecs didn't come on the scene until 1988 correct?
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Re: TPI fuel pressure issue

Postby 87vette81big » May 24th, 2014, 7:36 am

BOSCH 1'S Were used on all 1985, 1986, 1987, 1988 on all L98 TPI Corvette & F-bodys Bytor Stock.
L98 also used in a Few Caprice cop cars & Cadillac Devilles.
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Re: TPI fuel pressure issue

Postby grumpyvette » May 24th, 2014, 8:59 am

bytor wrote:I just checked out the TPI fuel system on my 87 vette the other day. I replaced the fuel pump because it was making a horrible noise. Turns out I had a kink in my return line right where the rubber hose connect to the sender. I replaced the pump anyway.

I did a leak down test. Got 44psi with key on and no start. It held but dropped to 38psi after 2hours. Is that ok?

Started the car and got a steady 38psi at idle. Removed the vac hose on the regulator and got 48psi so it seems my regulator is functioning normal. I checked the regulator vacuum line for fuel and didn't see any. However, I noticed a faint 'smell' of gas when I sniffed the vacuum hose that connects to the regulator. Also ohmed out all ma injectors. They all came in at 16.5 ohms. Question, on the 1987 vette, who's the manufacture of the injectors? Rochester? My understanding is the Multecs didn't come on the scene until 1988 correct?


that, pressure drop rates far BETTER than most, I,m happy if the residual fuel rail pressure stays above 38psi for 15 minutes after the keys off

normal

-98 Engine Start Sequence
When you start an L-98 engine Corvette, a series of events take place that causes the engine to run. Knowing the sequence will help you troubleshoot no start conditions.

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.

Initial Crank Action:

If you then rotate the key to the start position (assuming the anti-theft system has not disabled the starter), the engine will rotate.

Once the oil pressure has reached 4 PSI, the oil pressure switch will close allowing the fuel pump to run. (Note that you should have a black oil pressure switch/sender. It is mounted behind the distributor on the driver’s side and if it is not black, it is suspect due to a run of bad units that stayed in the GM parts pipeline for some time).

The distributor will send a string of pulses to the ECM (Engine Control Module) in response to the engine being rotated by the starter. These pulses continue as long as the engine turns (both starting and running) and if they are not present, the engine will not run.

ECM Reaction:

If the ECM sees oil pressure greater than 4 PSI and the reference pulses from the distributor, it will energize the injector drivers which will begin pulsing the injectors on for 4 ms (milliseconds) periods. (In the L98, all injectors on one side of the engine fire at the same time followed by all injectors on the other side firing at the same time. On the LT-1, the injectors are fired individually at the appropriate time).

The ECM will also pull in the fuel pump relay in effect paralleling it electrically with the oil pressure switch. (If the fuel pump relay fails, you can still normally get the car to start and run unless you can’t make at least 4 PSI oil pressure. This is a “limp home mode” feature put in place to allow for a fuel pump relay failure).

The ECM also monitors the TPS (Throttle Position Sensor mounted on the throttle body assembly) and wants to see .54 volts at this time. If it sees appreciably more than 0.54 volts, it will assume the engine is flooded and the driver has pressed the accelerator to the floor to clear the flooded condition and restrict the fuel flow as a result. (.54 volts during start and at idle from the TPS is very important to both starting and run performance.)

Assuming the ignition module is good (meaning there is a spark of sufficient intensity to ignite the fuel), the engine will “catch”.

Engine "Catches":

When the engine catches, the MAF (Mass Air Flow sensor mounted just ahead of the throttle body) sends a signal to the ECM advising that air is flowing and also just how much air is being pulled through to the intake manifold. The ECM takes note of the amount of air being consumed and adjusts the injector pulse width to around 2.2 ms nominally so as to attain a proper air/fuel mixture to insure combustion. (This is how the 1985 through 1989 L-98 works. For information on the 1990 and 1991 L-98 variant, see the Note below).

The engine should show an initial idle speed of around 900-1100 RPM and then slowly diminish to 600-700 RPM unless the air conditioner is on in which case it will run at around 800 RPM.

If this does not happen, the Idle Air Mixture valve (located on the throttle body) may be misadjusted. Alternatively, there may be a leak in the intake manifold or another vacuum leak may be present. Listen for hissing sounds---there should be none.

ECM Mode:

The engine will now be in Open Loop mode meaning that the ECM is controlling the air/fuel mixture by referencing values stored in memory.

Once the Oxygen sensor (mounted on the exhaust pipe) reaches operating temperature of several hundred degrees, the Manifold Air Temperature (MAT) sensor shows an intake air temperature of more than 140 degrees and the Engine Coolant Temperature (ECT) has reached 160 degrees, the computer will switch to closed loop mode meaning the Oxygen sensor’s output is examined along with the MAT and ECT outputs and the ECM adjusts the injector pulse widths (more “on time” or less “on time”) to constantly strive for a 14.7:1 air/fuel mixture which is the best mixture to hold down pollution.

Note that prolonged idling can force the computer back into open loop mode.

Note: In 1990, the MAF was eliminated from the engine in favor of a speed/density system. This system uses a sensor called the MAP sensor which measures the Manifold Absolute Pressure (hence the name MAP) and compares it with the atmospheric pressure outside the intake manifold. This information, coupled with the Manifold Air Temperature, Engine Coolant Temperature and Engine RPM is used by the ECM to determine the amount of air entering the cylinders. It is a different way of reaching the desired 14.7:1 air-fuel mixture ratio but functionally is like the MAF system in that the ECM uses the feedback to control the "on time" for the injectors.

Corvette used this approach in the 1990 and 1991 L-98 engines and in the 1992 and 1993 LT-1 engines. With the 1994 model C4, they went back to the MAF system. Note that MAF based systems are far more accurate since they measure air flow directly whereas the MAP system infers air flow indirectly. A multitude of things can throw the calculation off and Corvette returned to the MAF system beginning with the 1994 C4 (with a MAP backup). From a troubleshooting standpoint, the MAP operation comes into the sequence the same place that the MAF does.

Summary:

If you have a no start condition or if the L-98 starts and then dies, check the above items in sequence to see if all the events are occurring as required.

A Scan Tool makes this job much easier and is a highly recommended troubleshooting aid for these sorts of problems.
youll find a hour or so reading thru the links and sub links, on this site in the threads, will provide a great wealth of related info and incite into related factors, or the function or testing of sensors, that you may not currently be thinking about, or things that you might not think that are related to your issue that PROBABLY ARE
IF YOU CAN,T SMOKE THE TIRES AT WILL,FROM A 60 MPH ROLLING START YOUR ENGINE NEEDS MORE WORK!!"!
IF YOU CAN , YOU NEED BETTER TIRES AND YOUR SUSPENSION NEEDS MORE WORK!!
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