Good evening gents,
I feel a bit embarrassed sometimes as I keep leeching here but can not provide any help yet. I will learn more and hope to really lend a hand or some thoughts to new owners one day.
So my 1971 series 1 xj6 had just covered the first 1000 miles after I took ownership six months ago. It runs a 4.2lt equipped with twin SU HD8 carburettors.
I use it mainly on mild hillclimbs in the weekend, doing 20~50 miles every week. She has been running very smoothly since I bought her, however an issue of revving difficulty and power loss happened recently and I have to keep her in the garage for now, which is not ideal especially in our autumn here, which I believe is the most beautiful season to drive through the hills and valleys.
This problem happens only when the engine is warm/hot and it is getting worse as I use the car more. The first time when it occurred, I was doing a long hillclimb. The big cat suddenly lose power while climbing, and the engine refused to rev higher than 1500rpm even with the accelerator floored. Shifting it down one gear temerally increased the rpm to 2000 but dropped back to 1500 in 2 seconds. I then turned around and went downhill. The car had no problems making the 20 miles trip home. The second time when I encountered the same problem, I was on my way to a weekend valley drive. I had to drive the car in limp mode into a shopping centre carpark, waited for about 15 mins (I thought it was overheated) and drove home without a problem. After that, this power loss and rev hesitation problem got worse. I was only able to do a few loops around my house before the engine started to struggle. When the car is in gear, flooring the accelerator can only keep the rpm @1500~1800 maximum and the engine feels much weaker than a normal running xk engine at 1800rpm. If I don’t keep my foot on the gas pedal, rpm will drop to lower than 500 and engine would almost stall. There was no smoke. Engine was not overheated. And it could be restarted straight again showing a normal warm idling at about 850rpm. Until I put it in gear other than “P” or “N”, rpm dropped and the engine struggled again.
I searched earlier posts in this forum with filters “stalling” “power loss” “heat” “warm” “hot” to go through the related information. There are some suggestions from you more experienced owners, including the following:
- Electronical problems：may originate from ign amp, coil and distributor which can be affected by heat.
I have replaced all spark plugs recently but can not remember if I did it before or after the problem occurred. During one episode of power loss, I parked the car and touched the coil and found it to be cool. Engine speed did not drop violently either. Just the engine felt much weaker at the same rpm.
- Dirty air filter: One PO suggested a very dirty air filter could play a role if the grease blocks the air flow badly. I had the air filter replaced and the problem was not solved.
- Problems with fuel system: More discussion was found on possible fuel starvation. There are two fuel filters in the car. Fuel filters #1 in boot and #2 before carbs. I am not sure about the tanks condition. The previous owner did mention he experienced fuel delivery problem due to dirty tanks, he then changed them and had new liners put in tanks with receipts shown in the folder. I tried to switch tanks several times when the power loss problem happened, however there was no noticeable improvement.
- Blocked catalytic converters: One owner suggested the contribution of blocked cats. I am not sure with the condition. I placed a hand over the exhaust pipe and I could feel strong puffs of gas.
- Oil pressure problem: In one post, oil pressure problems related to blocked breather was mentioned. I opened the breather, found thin coat of dark oil, but the F tube going to the intake flows freely.
- Carburettor problems: one owner had similar problems but all solved after a carburettor service.
To summarise, the engine loses power (has rev difficulty) after various driving distances – this distance is getting quite short now. I did not notice any misfire/bang or smoke when the power loss happened. It is more obvious when the car is in gear other than “P” and “N”.
It happened only when the engine was warm. The engine was not overheated as indicated by the temp gauge also by an IR thermo gun.
If I only start the car in garage to warm up the engine, such a problem has not been observed in some 20~30 mins period after the engine is operating in normal temperature. The engine can rev at 3000 rpm freely.