When I first got my car a week ago, I noticed a slight miss when you applied gas to speed up at speeds of 55 & above. Thought it was the computer program problem I had read about on line (goes away at higher RPM) so I just ignored it.

As I drove at higer speeds with the cruise on, the car ran smooth until I tried to pass and stepped down on the gas. Then it started bucking and missing. It seemed that the longer I drove the worse it got.

This would not be a computer re-programing problem but a fuel or ignition problem.

Now it seems that the car doesn't miss at around town speeds and on short runs.

Any ideas? car is a 1998 XK8 with 90k good air cleaner and good gas (I think). that is all I know about it so far.

The car was on empty when I picked it up and had to get gas a a local no name station. Poss. water in the fuel? I have since added more fuel from better sources, maybe the reason it is running better now?

Or the problem is a bad Air flow meter that gets worse with speed and time and engine heat? or faulty coil pack(s)? worn Plugs? cracked coil pack rubber boot(s)?

All ideas welcome.

 

Check engine light or any codes?

 

No engine light, that is one of the reasons I am not sure about the Air flow meter. I am pretty sure it would throw a code & ck engine light.

I am taking the car to my Auto doctor on Monday and he is going to look for codes etc.

 

Weird that you have no check engine light, but I'm guessing that it's a fuel delivery issue. Either a fuel pump or an extremely dirty fuel filter. An air delivery problem wouldn't cause the problem you described because the ECU uses the MAF to calculate how much fuel to send to the injectors. If there was an air problem, the MAF would just see less airflow and you'd have crappy acceleration, but it wouldn't buck and miss. A fuel delivery problem, however, would cause the symptoms you described.

 

I checked the air filter, it's new. I guess Monday will tell the tale.

I have a new fuel filter on the list of things I want done, including new plugs and check the boots for cracks etc.

I have also requested a complete ignition & fuel test as well.

Thisw guy is really good and I know he will find the problem and I will be on the road again!

Thank you for the help

 

Well, It could be a Air problem. A cracked, leaking Air Tube would allow air to bypass the MAF, resulting in a Lean Mixture.

The Air Tube can develop cracks in the accordion folds as it gets brittle with age.

 

Could also be a failing ignition coil, I had similar experiences and also no codes.

 

at highway speed, those symptoms usually point to the throttle body, either the connector harness or even the throttle position sensor (more rare)

BTW, dont ever spray down the engine these cars are very prone to moisture issues on a few harnesses, namely the TB. Even driving in the rain can cause enough steam and condensation to get into that plug, s-types and xks are the most receptive

 

Turns out my idea of water in the gas wasn't to far off just in the wrong part of the car.

Seems that when the car was detailed, the engine was cleaned and water got into the area that the coils and plugs live. OPPS!!!!! Need I say more?

I guess as the water got hotter and then became vapor, the coils jumped spark under load at speed causing a dead miss at speed.

Problem solved. Water is gone and so is the miss.

Thank you for all the ideas, they were all good and I am so glad I didn't need to buy coils or a throttle body.