My car has been running great for the last couple of years, with several very long trips behind me I have had no issues. The other day I was on the highway for an hour and noticed while at a steady pace on smooth roads at 70-75mph it started surging, ever so slightly, but enough for my wife to notice and she finally asked about it. It was almost as if I was letting my foot off the pedal a bit and back on, and was at a frequency of off for 1/2 second, on a second or so, then off a half second again, and so on. But it was very subtle, not fully off, just as if I slightly let up on the gas. But I had the cruise control on. And so I turned off the cruise, kept my foot steady, same issue. RPM gauge was rocky steady, so I don't believe it is the torque converter, as I would see the engine speed change. Step on the gas to accellerate - perfectly fine, no hesitation. Arrived at our destination, parked car for a few hours, then drove home, same thing was happening. It didn't happen constantly when on the highway, but did go on for about 20 mins steady. Non steady speed driving, such as around town etc, perfectly fine. A few days prior to this I noticed this on a 3 mile road leading to my house that is smooth and steady speed of 45mph. Then the next day I filled up the tank (had about 1/4 tank, so not too low), so I don't believe it is bad gas. No codes. Fuel pressure is steady and normal. When this was happening the Coolant temp was at 187, outside temp was 78. Fuel is 92 octane. Fuel filter is a year old. Spark plugs are about 2 years old with maybe 30k max on them. LTFT at around +4 to +5 consistently. I did not take any live OBD measurements while experiencing the surging. Once home, I measured live data, and suspecting a vacuum leak as a possibility I used H2O Boy's find a vacuum leak method with TB cleaner watching STFT and in essence I felt that my tests were inconclusive as I ran out of TB cleaner before being fully confident. I did see what seemed like minor fuel trim drops, from 2 to 3 dropping to -3 to -5 when spraying around the intake manifold, and around the corrugated area of the cold air intake, but not every single time I sprayed. Only one time, when spraying the front metal plate on the intake manifold did I see it drop all the way negative then bounce back up. I thought eureka, there it is, but I sprayed sever times again in the same area, and never saw it budge again. Any thoughts? Would minor vacuum leaks cause such surging? Any other ides?

 

I am consistently having this issue now when I drive the car, noticing it most on steady speeds and RPM's from just under 2k to 3k (of course that is where the typical engine speed would be at steady speeds). I was on a straght open road at 40 or so MPH yesterday and while experiencing the issue I datalogged several OBD sensors. I am not sure if there is a correlation to what is shown, and to the problem, so I wanted to share here and see if anyone see's anything I don't. And also, I am not sure if the fluctuations I see in the sort term fuel trims are normal, or if they would even be noticiable when driving. To that, the only thing I see as a possibility is the STFT fluctuations. I have uploaded to youtube the datalog video. It shows the STFT fluctuations and also shows a steady RPM, steady MAF, steady fuel pressure, steady load. You will see RPM's drop distinctly 2 different times, and I used the J-gate to maintain engine speed at 3 different points to see if it made a difference, but I am not adjusting car speed, and gas pedal is steady throughout. As well, I noticed here that my LTFT for both banks are slightly negative. The last time I looked at these they were slightly positive, at around 4 to 5 or so, which was about 6 to 9 months or so ago. I would like to hear any ideas or thoughts anyone has, and I don't want to plant any ideas, but will: the only thing I can think of is a bad injector (or several), that maybe are "leaky" or dumping too much fuel. I know injectors can leak, but I don't know if they would exhibit such symptoms as surging. I am thinking a leaky injector dumps too much fuel, so ST and LT Fuel Trims go negative (like I see in the data log), and the fluctuation of the STFT might be the ECM trying to compensate for this? Has anyone has this happen and it was injectors? How can injectors be tested for leakiness?

https://www.youtube.com/watch?v=Z3zQ...ature=youtu.be

 

Closely examine those accordion pleats in your plastic air intake duct. Time and heat both take their toll and eventually crack and split the pleats, allowing hot engine bay air into the airflow. The cracks can be difficult to see unless you flex and bend that accordion section....

My wife's 2006 XK8 suffered from this problem in October 2012. A new plastic air intake duct resolved it. I negotiated a price of $125 for the new part from the dealership, but you may do better sourcing it from our forum sponsors now....

Don't stick a used one in there. Its lifespan will be greatly reduced....

 

Thanks Jon. This was one of my first thoughts, and I spent quite a lot of time trying to dignose it by spraying with TB cleaner all around the pleated area while watching the STFT's. The results were inconsistent. I would spray, and then see the STFT drop, then rise again to previous level, spray again in the same area, no STFT change, spray again, drop, ect. I really did think I had it confirmed at one point.

My question to you, were you experiencing the same surging?

 

Only occasional surging at highway speed, perhaps every few days or so. Most of the time the engine behaved normally. Keep in mind that she drives it, not me. So I have to go by what she told me....

 

At first look, the airtube appears fine. As I said, I was spraying it with TB cleaner and watching of STFT changes, which was inconclusive. I never saw any obvious issues with the appearance of the tube. Upon advice here, I pulled the tube out, inspected closeley. I still really didn't see anything. I have an LED light about an inch long by an inch wide, very bringht. I put that in the tube and went into a dark room, saw light coming through. Now I know where to look, and I took some colose-up photos, one attached here. It may look obvious, but these are close up shots, and I assure you this is not at all obvious. I duct taped the area to test ONLY, but will now order a replacement regardless of if the tape works or not, I just want to know if it makes a difference and if this is the root cause.

 

Here's hoping that you've found your problem.

 

This has got to be one of the best forums in the world. I'm very lucky in the fact that I haven't experienced any problems with my car (yes, I'm knocking rapidly on wood) except for the intercooler aux. pump. I read these posts and try to file these challenges in my head knowing that I have a great cast of folks that can lend a helping hand. Thanks Forum!

 

I have experienced the same type of surging, at first I thought it could be the throttle position sensor. The only difference to the surging in my 97 is that it would not do it when cruse control was on. I have adjusted the throttle cable, cleaned the MAF sensor and cleaned the throttle body and butterfly off the car, since then the surging has not returned.

 

I replaced the intake tube, cleaned the throttle body with TB cleaner, cleaned the MAF with MAF cleaner, changed the spark plugs, no change. I have checked again for vacuum/intake leaks with starter fluid spray while watching STFT live, I am sure there are no leaks. I have watch as many engine sensors and settings as possible live while experiencing and see nothing unusual. Fuel pressure fine and steady, ignition timing seems steady, STFT and LTFT seem normal. MAF reading are steady. I don't know what else to look at. I am leaning toward an injector problem, or Torque Converter. Neither of these do I know how to diagnose. Any other ideas?

 

I have resolved the issue by replacing the fuel injectors. As mentioned, I had suspected them as a possible cause, yet really didn't know how to diagnose them, other than if they were leaking. So I went onto Rock Auto and bought 8 rebuilt injectors. Car now runs shockingly stronger and smoother, even though anyone would have thought it ran perfectly fine before (except for the occasional surging). I suspect that I probably had at least one injector that was worse than others, maybe internal junk occasionally getting stuck inside blocking or slowing fuel, idk. When looking at the nozzle holes on one, some of the holes don't look clear. I would bet that with 100k miles the spray patterns and flow across all 8 are probably not matched and not up to par. I feel that the approximately $240 was money well spent based just on the way it runs now. Oh, and a very very slight/subtle and very short lived engine shake on cold starts is now gone. Again, this was very slight, which most people may not have noticed.

 

SteveJacks,
Soon after changing my trans fluid and pan I started getting the 1-200 rpm range surge while cruising. I've read maybe torque converter problem, or flashing the TCM/ECM needed, but apparently the fuel injectors solved your problem with the surging.
Associated with a P0420 code I had my Jag dealer run diagnostics. They recommended spark plugs and fuel injector cleaning. I put new plugs in, which didn't solve my problem, but I have the RockAuto GB Reman fuel injectors supposed to be arriving tomorrow. So I'm hopeful that will clear up my surging problem as well.
Did you put them in yourself, and any advice on doing so?
Thanks,
Kenton

 

I think your code is a catalytic converter efficiency error. If I am correct about the code, then if would consider first that maybe one of the O2 sensors may not be working properly. The are 2 o2 sensors per bank, and as I understand it, the lower sensor expects to "see" lower emissions than the upper, which assumes that the catalytic converter is working properly. So, either one of the converters is no longer working well, or an O2 sensor may be failing. At least that's where I would start. However, I could be wrong on the meaning of the code, others may have input. If it is the catalyst efficiency code, then either old age / high mileage could be the issue, or maybe other issues causing catalyst damage, such as overheat, poor or incomplete combustion, etc, which would need to be diagnosed and resolved prior to catalytic converter replacement.

Anyhow, I replaced the injectors myself, fairly easy to do, taking care to properly lubricate o-rings, and taking care to make sure the injectors are well seated, as you cannot rely on pushing down on the fuel rail to seat them, you need to be careful making sure they are well seated, firmly. Fairly simple job overall. I Installed the new injectors into the fuel rail, then set the rail down in place. If you push down on the rail to try and seat the injectors you will be flights go the rail mount. I had to push each injector into its seat and it is tough with your hands in there, you have to avoid popping them out as you do one and move onto the next, but take your time, not too bad. Keep in mind, I had no such codes at all.

 

Believe me, I've been around the block with different mechanics and threads on this forum about the P0420 code, with all the various possible causes and scenarios. I do want to resolve the problem of what's causing it rather than replacing parts that will at least temporarily remove the code (such as O2 sensors or cat replacement).
But I'm mainly intrigued by whether the fuel injectors are the cause of the surging. Thanks for your advice about how to replace them. Unlike some other vehicles, the fuel rail does not appear complicated to get to. I'll be very happy if indeed it is the injectors and not a transmission problem.