5.7-89GTA

Looking for some input on the issue with this car. I've had it for 12 years. It is a stock L98 (except for cat delete). Haven't really driven it for about the past three years - it kept burning up cat converters and I suspected it was running rich because the original 30+ year old injectors may be leaking. However, I did not perform any tests other than to ohm check them and they were within specs. But, over a COVID winter I decided to swap in new ones and got 22 lb Delphi injectors from Fuel Injection Connection. I also replaced the regulator with a stock replacement from FIC. The car fired right up, except it kept setting a SES light with codes for O2 sensor and MAF. I replaced both (the MAF is a rebuilt Bosch from NAPA). This corrected the code issue and the car ran better than ever, for about 2 weeks. It would start right up, but stumbled and hesitate when cold and would not take throttle. When warmed up it seemed to run OK so I suspected the CTS and replaced that. Runs better cold, but still will not take WOT on the freeway when fully warmed up.

I replaced the fuel pump and filter about 8 years ago (and only have put on about 1500 miles since that job). With my crappy Harbor Freight fuel pressure gage I am getting 42 psi at key on (engine not running) but it bleeds down quickly after the pump times out. With the car running I am only seeing 35 psi and about 37-38 psi with the vacuum pulled off the FPR. I also tested the system by clamping off the return line (engine off!) and saw an instant 70 psi and it held for as long as the clamp was on so I do not think I have any leaking injectors or anything else on the feed side. As I loosened the clamp the psi dropped down to 20 psi in about 30 seconds. What could be causing the FPR to not reach the optimum psi and why is it bleeding down so quickly? I was as meticulous as I could be when I replaced the unit. There is no fuel odor and my clamp test means to me that it is bleeding off into the return. What am I missing here? I know now what a PITA it is to work on the TPI fuel system. If I need to R&R the FPR, how much of a pain is it to replace without removing the plenum. Is the low psi the likely reason for poor WOT response?

Is there anything else I should be checking/testing? Many thanks for any assistance with this.

NOT A FUEL PRESSURE ISSUE. TRIED TO CHANGE TITLE OF POST. SEE POST #12.

Johnny Gun

FPR is simple. Just be sure to relieve the pressure at the schrader valve first.

Vader

Your pump can maintain 70 PSI with the engine off. If you restrict the return line while the engine is running, can the pump still maintian at least 50 PSI?

As for access to the regulator, it is far easier to remove the plenum. If the gaskets are not 30 years old and the fasteners were antiseized and only torqued to specs when it was assembled, it might not be that difficult to free the plenum.

5.7-89GTA

I just reassembled the complete intake in February with new gaskets/antiseize/correct torque so removal should not be that difficult. I will try the engine running with return clamped to test psi. Thank you for your reply.

5.7-89GTA

Today I re-checked the FP with the return clamped. I must have done a better job of clamping the return because I got 80 psi. I ran the engine for about 10 seconds with the return clamped and the gage would bounce between 80-90 psi. However, I noticed a drip of fuel coming out of the fitting at the gage end of the line, so I may not be getting accurate readings. I will retest with another gage to verify. But, I still have 42 psi key on and 35 psi engine running without return clamped. 37-38 psi with vacuum off.

Vader

35 PSI running is only just a bit below the specification, and possibly within the accuracy tolerance of the gauge. With no vacuum applied to the FPR 43 is more normal, but again may be within the ± accuracy of the gauge. If the FPR has a problem, it isn't leaking past much more than what normal would be Certainly the pump itself is getting the job done at idle. The next queston may be whether the pump can maintain that kind of pressure under the demand of higher flows.

5.7-89GTA

I used an Auto Zone FP kit today. 42 psi key on. 37-38 psi engine running and 42 psi with vacuum pulled. I also noted that bleed down was much slower than my HF gage. My gage is 3-5 psi lower than the AZ gage. Revving the engine sees a slight rise in psi before it dips a little.

Car will rev freely in neutral. From a stop it will rev hard to 4500 rpm in first gear until shifting to second. I run out of road after that. On the freeway, cruising at 60 mph and flooring it the engine struggles to reach or exceed 3000 rpm. No SES light.

Not so sure I have a fuel problem. But there is a problem - could it be ignition or transmission? I changed out all of the ignition components when I got this car in 2009. I do not have any data logging experience or equipment. I may borrow the AZ FP gage again and road test the car with it attached. It is a nice unit with a long hose - I could probably tape the gage to my windshield. Any suggestions appreciated.

Vader

If fuel pressure and flow are sufficient to support driving the vehicle in 1st gear to 4,500 RPM, that should be adequate. Your updated pressure measurements seem to reinforce that. Taping a pressure gauge to the cowl or windshield would remove any doubt.

Does the trans downshift properly when going WOT while cruising? At 60 MPH the TCC should unlock, the trans should shift at least to 3rd, and possibly 2nd until the throttle is lifted a bit.

Ignition, including timing advance and spark energy, are also among the suspects. Spark blowout under higher dynamic compression pressures can occur. Weak wires, coil, worn cap and rotor can contribute to that, as well as incorect plugs. If misfire is experienced under heavy acceleration, that could be the case. It may also be worthwhile to set the crank at TDC, document the position and orientation of the distributor cap (specfically the terminals) and remove the cap to observe the position of the rotor. If the rotor is not within about 15° of the center position of the cap termminal, the distributor may need to be reinstalled to obtain proper orientation, The firing of a cylinder with the rotor between the cap terminals is not a good situation.

Another possibility is late valve timing. That can be diagnosed externally but the other investigations should be performed first.

A restricted exhaust is also possible, but less likely since it will evidently hit 4,500 RPM under load.

5.7-89GTA

Thanks for the detailed reply. What confuses me is the performance in first gear vs. the highway downshift scenario. I am going to verify FP on a road test sometime next week. After I got the car back together this Spring it ran great, especially after the SES codes were corrected. On a clear stretch of highway it ran strong up to 100 mph.. Then, over the past few weeks the performance has deteriorated to the current state. Whatever it is eludes me and FP was my first suspect.

ploegi

Cat plugging up?

5.7-89GTA

Car has no cats. I forgot to report in the OP they went missing.

5.7-89GTA

I got the AZ FP gage kit again today, taped the gage to my windshield and went for a test ride. No loss of FP at any throttle setting, including WOT. Stayed between 38-42 psi. I was unable to duplicate the WOT problem that I reported in a previous post. Car was up to operating tempurature as I tried some WOT from 60 mph. Would downshift to third and I had to let off around 4000 rpm due to traffic. The car seemed to operate normally. I don't mind the car fixing itself, but when you don't know why and you are expecting the problem to arise again at any moment that is frustrating. I will go back and check my ignition components and the other suggestions made to this thread. Thanks, VADER. Looking back at some other posts, you have been doing this a long time and your expertise is greatly appreciated. (Also- with the AZ gage properly attached I had very slow bleed down which tells me my HF gage is faulty.)

I thought there was a way to change the post title. I don't want to mislead anyone trying to solve a FP issue only to find out in post 12 that was not the problem.

Vader

Thank you for the update. The fuel pressure seems to be normal under the conditions you describe - At least for now. There is a remote possibility that the FPR managed to get slugged with a flake of rust or contamination. Hopefully the continued operation will trap all that in your filter and eliminate the potential problem.

Whether it "fixed itself" will be revealed in time, but at least you can probably focus somewhere other than the fuel pump if problems arise again.

You should have been able to edit the thread and change the title in the process. I could.