Speed Drops at High Speed

[iamfaizankhalid]

I have a Civic 99 Automatic. The Issue I am facing for the last few weeks is when it is moving above 100, and I push the pedal, and it reaches around 120, suddenly the speed drops as if the car was put it neutral. RPM remain the same. And after a few second with a bump, speed start to increase again as it was put into gear again. What can be the problem?

[Massive_One]

there are two possibilities,

either not enough fuel is supplied at higher speeds - hence the engine struggles at higher speed and coughs.
or there's a disruption in electrical supply to produce enough sparks in the engine at higher speed..

get your fuel pump and injectors tested
get your electrical wiring and fuse tested

[Xulfiqar]

So when you say car becomes neutral, that means the rpm will be shooting high and vehicle speed will not.

Is that the case?

[iamfaizankhalid]

No. RPMs remain the same. Just the speed starts dropping.

[capsat]

what is vehicle maintenance history in terms of Transmission fluid change?

[Xulfiqar]

so then its not in neutral - you probably are facing a speed limit due to the ECU being stuck in limp mode and some shenanigans have been done to fool the driver (check light bulb connected to something else etc).

Abt close to 3500 or 4000 rpm is when the vehicle reaches 120 kph - and that rpm range is exactly where the fuel cut happens.

If your engine, ecu, its wiring, its transmission original to the car or has some "cheetah" assembled it together with "jenyan kaabli parts?" and made it teetarr batair?

[iamfaizankhalid]

Yes that can be the issue. Until a few weeks ago, the engine couldn't rev up more than 4000, and the speed didn't go above 140km/h, no matter how hard I push the pedal. The issue never happened back then. Then I don't know if the mechanic has done something or as he says he didn't touch the engine, the car can now revs higher and I have took it to 145 (not further due to traffic).

Also, there is another issue. Sometimes after driving for some time at 60+, CEL appears randomly. I have Hondash scanner attached to the ECU, it gives code P0500. But if I turn the engine off and on again the light disappears.

[Xulfiqar]

here you go, your issue presents itself, check if the speed sensor is flaking out. missing VSS will cause the ECU to go into limp mode.

[iamfaizankhalid]

But the speedometer show the speed accurately. And if I clear the code, it won't show again until the next time it pops up. Maybe it is just loose.

Also, car shift gears perfectly.

[Xulfiqar]

your cars automatic transmission is fully hydraulic, it has a speed governor in it that runs off the differential assembly which dictates the up shift timing against the pressure cable input. - Meaning the VSS has no input for it regarding shifting gears - it however has one input which is the TCC lockup signal.

If your car has ABS then the diagnosis is slightly skewed because in the ABS models, the speedo is driven by the ABS pump as average of all 4 wheels - your VSS is used by the engine ECU only.

If your car has no ABS then your VSS is shared by both speedometer and engine ecu, meaning the wiring fault can be somewhere in the dashboard wiring (usually near the ecu itself)

[iamfaizankhalid]

Okay. I will get it checked. Thank you.