I have a 1999 Ford Laser KN 1.6L manual, which I have owned for about 8 years. I’ve kept it up to date with oil, filter and spark plug changes etc.
Over the last few months it has started to become a bit jerky when under light throttle (cruising) or no throttle (coasting). The jerking feels like I’m randomly pressing the brakes slightly, but I’m not. Its definitely not cylinders missing (engine note stays smooth and I have another car that misses, and it feels very different). I don’t think the brakes are to blame because the car doesn’t have any electronic brake control (not even ABS), and if I put the car into neutral, the jerking goes away. Full throttle, the car still has full & smooth power, and the idle is smooth, although it idles higher than it used to.
The problem has been getting slightly worse over time, having started a few months ago. The other weird thing, which may or may not be related, is that recently the car has sometimes (not every time) been taking a long time to crank before it starts up.
Any ideas? I’m wondering if one of the engine sensors is faulty, although there is no CEL.
The best way to find the problem is to plug a scanner into the car, drive it until you feel it and take snapshots of the data off the engine until you find it.
What you describe sounds like an engine miss. But not bad enough to set a CEL. Weak coils or injectors maybe. A scan would help pinpoint it. If the long term fuel trim is high, could be a weak fuel pump. If low, could be a weak coil.
My older OBD I Corolla developed a similar symptom noticed mostly in low speed driving, like 25 mph stop & go in neighborhoods. Discovered the primary problem was a misadjusted throttle position switch. A dose of fuel injector cleaner helped a little too. So if I had that problem, that’s where I’d start: verify the throttle position sensor is working correctly and add a dose of fuel injector cleaner to the gas tank.
Beyond that, an O2 sensor failure could cause that symptom by forcing an overly lean mixture.
The fuel trim test suggested above seems a good idea too, could point to different culprits, and pretty easy to do for a shop.
You may have an unmetered air path into the engine, causing it to run overly lean. The fuel trim test would confirm or disprove. If the engine is running too lean, likely possibilities are fuel pressure too low, faulty vacuum hose or vacuum actuated device, engine intake air boot is leaking, among other possibilities. Before going down that path, suggest to first do the fuel trim test.
It seems like Fords of that era had a problematic EGR modulator, had a 4-letter acronym starting with "P"as I recall. Good idea to ask your shop to check that too. Another idea, if your engine uses a fuel pressure regulator located on the fuel rail, remove its vacuum line. There should be no fuel in that line. If there is even a drop of gasoline, the FPR is faulty and must be replaced.
Appreciate the suggestion, but as I said I’m pretty confident its not a miss - I know the feeling of a missing spark, and its a different category of ‘jerk’. I’m not sure what other word to use, but its a very gentle ‘jerk’ - like very lightly pumping the brakes. I mentioned it once to a passenger and he said he didn’t even notice it, so its very subtle. I only noticed because I was in control, and know what the car ‘should’ be like.
Unfortunately this car doesn’t have an OBD2 port, otherwise I would check this with my scanner.
Clearly this is not a US car or it would have an OBD2 port by requirement. There should still be a data port in the engine compartment. Finding a reader is another matter.
A miss is the only thing I can think of that would cause what you are feeling.
The light surging is probably cause by the idle air control motor “hunting” for position. Since you are experiencing other idle speed symptoms, replace the IAC motor.
Well believe it or not, the car started misfiring today. Definitely running on 3 cylinders now. I think I will start by replacing spark plugs - any other root causes that could have been jerky deceleration that have now developed I to a full misfire?
If you turn off the ignition while coasting, does the issue go away? I suspect the the braking feeling is engine braking. Some cars turn off the ignition completely when coasting. Maybe your engine is turning off and on. Or IAC as stated above. I think normally IAC wouldn’t be adjusted unless it is at idle speed.
Yes, the issue goes away if I put the car in neutral. It feels like engine braking to me - and I thought it was the IAC, except that last night it also developed a misfire! That has confused me a bit. Unless there are two things wrong coincidentally
