I have been fighting an issue with my 1986 XJ6 series 3 in which at high revs, such as really flooring it and passing (above 3000RPM) will cause the car to stutter and lose power and no longer accelerate. This happens at 65/70 mph when flooring it downshifting to 2nd, and the car will not be able to switch back to 3rd under load and lose power until I ease off the throttle, then I can gradually speed up. If I stop the car and floor it, It will get so bad that I can't make it to 30 mph-huge hiccups and backfires.
I have been addressing vacuum leaks this week which I thought might be or contribute to the issue and have those fixed I think. So I set out to test today. Initially the car was fine, but I sought to torture test. The problem started to appear from a dead stop floored up a hill. The car at 3200 RPM 35-40 mph could not shift or accelerate. I opened the gas cap and same result. I switched tanks cap on and off, same result -fail. I did each 3 or four times and collected my thoughts...tried again and the car performed perfectly. what? I took the car to the highway to see if the problem would pop up in passing gear. At about the 5th attempt all seemed functional and well. Punch it at 55/60 mph the car downshifts (it is automatic) accelerates revs to 3400+/- and shifts. The 6th time....gasps and cannot shift or accelerate past 60 mph. now I keep at it and cannot get it to reach 35 mph if I floor it. I drive slowly, gradually accelerate and get back to my garage. I quickly install a pressure gauge into the cold start position on the fuel rail. Start the car and read 31 psi. revving brings pressure quickly up to 39 psi and back down at idle. I fasten it to the hood and set out.
The cruising psi is 31 psi, hard acceleration raises it to 39 psi no more. The car behaves well...for about 15 minutes of torture test. Then as usual the cruising psi is 31, but I go to passing test, and the pressure starts dwindling down to 20 psi. a repeat is really taking the pressure down now. I pull to a stop (still running) and it has trouble with basic acceleration, the fuel pressure easily being sucked down to 4 psi. But always returning though slowly to 31 psi. Both tanks.
I get the Idea it may be a suction side hose delaminating/caving in, but it can only be the hose in between the crossover and the pump, because the problem is present irrespective of tank sides. The fuel pump makes a hell of a racket whether the problem is presenting or not, and the pressure side actually vibrates. Is this normal?
I have cleaned out the tanks a few days ago and they were pretty rusty. I have a clear filter in line on each side before the changeover valve. There are a few flakes but not occluded. The in tank baskets have been removed from either tank. The filters are 3/8 inch as that was the biggest I could locally buy. The hoses from the filter to the crossover are 3/8" not what looks like the 1/2" from the tank to the filter, could that be a problem? I thought not as the metal line forward looks to be even smaller! I have a new line to replace the crossover to pump.
I am going to autopsy the old one for collapse. But if not that then....Does this problem sound consistent with a failing fuel pressure regulator? I don't know if rust made it that far past the stock filter, but... the car sat for 20 yrs before me. Any help/ideas appreciated! I will report back after test drive and autopsy!

 

Bad fuel filter. Engine starving for more gas !!!

 

 

The two (one per tank) pre-crossover valve clear filters and the large jag filter are new. The problem is intermittent. Merely turning the engine off seems to reset for normal driving. My autopsy revealed no collapse in the line between the crossover and pump. I replaced it with a new 1/2" line. I just removed the air bleed non return valve to check for debris that might block it; is there nothing in it but the tiny valve to air and tiny valve to fuel rail in the fittings that screw in? Would fuel pressure regulator be intermittent like this?

 

 

The problem occurs with either side selected, crossover checked and functioning. My pump is in the trunk, after the crossover and before the big fuel filter. Maybe your issue could be related to the return valve in the rear wheel well freezing? Try turning on the fuel pump and listening at each fill hole for the return gas pouring in at the proper tank after you switch sides in the cab.

 

IMO, don't do anything until you've checked/replaced the fuel filter. Empty the contents of the filter into a clear jar. What you see might dictate your next steps.

Cheers
DD

PS- using paragraphs makes long postings much easier for the reader to read

 

So great, the two clear filters I bought a week ago at the local auto parts store for monitoring tank rust filtration prior to the crossover valve have both failed. The filter is glued in, they both had come loose. I clean everything out and re do the filters. I have also snugged up the vacuum line to the fuel pressure regulator. I spark it up. 35 psi instead of 31 at idle. I take it for a test. i put my foot to the floor from rest. in first gear, at 3000 RPM it starts sputtering and coughing gasping can't make 35 mph but....the fuel pressure is fine! 35 psi. I try this a few times. What the hell?!
I turn the car off to think. I start it back up to go to the parts store...cannot get it to reproduce the problem. It runs Fine. I give up for the night. Anyone have a clue? I am going to the garage now to watch under the hood at night for arcing. Ok, that revealed nothing.
The last gasping spell completely confounds my thoughts. The pressure was fine. Measured with an analogue pressure sensor from the rail while driving, during the episode. The car was not yet hot, only a matter of minutes from the house at low speed. The spluttering made me suspect electrical issues. The problem is catalyzed at high RPM's, at low speed/revs it behaves fine.
I tried jiggling the key for ignition switch problems and a few different micro positions to see if it made a difference-nothing, ran fine.
Sorry about the run on Doug, I'll break into paragraphs. I am striving for accuracy and to include details that might be relevant. I'll aim to be concise. I have been thankful for the information in so many posts by the folks here on the board and hope that this may help someone in similar straights....that is if I can figure it out!