I need help from the pro's. 97 neon sohc. Suddenly started surging like everything was interrupted in 3rd gear, maybe binding in the tranny or loss of fuel or spark. looked down and speedo not working. I changed the plugs, oil, air, checked vacume hoses, wiring etc. followed haynes and changed out the vehicle speed sensor, when I pulled it out of the top of the tranny, the plastic gear cone thingy dropped down and I put a new sensor in, but I noticed that there was nothing in view inside the hole for the gear on the cone to turn against. Common sense tells me there should be and maybe whatever is supposed to be there is now getting chewed up and causing the surging in 2nd and 3rd gear every once in a while. only happens after warming up and driving a while. motor seems to idle and have plenty of power. I drained the mtl and screened it, no particles or pieces of plastic gears or anything. Is there any tranny gear heads in here that can give me some pointers. at this point the only thing I can think of is to just drive it until it breaks since the tranny is about 250 local wrecking yards. Whaddaya guys think?
Jim

 

guess I should add it is MTX, and the VSS changeout did not fix anything, and no CEL at all throughout any of this. The surging comes and goes and no matter what I do I cannot get it to repeat the problem with any consistency, just does it when it wants to, usually in 3rd, sometimes in 2nd.
thanks

 

How many miles on the clutch?

The TPS and Crank Sensor can both cause surging/lurching without throwing a code. TPS is much cheaper... Without a CEL, it's hard to know where to start...

Best of luck!

 

The clutch and Bearing are new, just put them in about 2K ago. Everything was running perfect afterwards. It doesnt feel like a slippage, it is more like if you were to stomp on the gas and bounce your foot on and off the pedal at about 3K rpm, the car shudders back and forth. I guess either I loose all spark or all fuel for a split second a few times, then the car is fine. It still has not completely stalled, and when I pull over off the road, everything is fine and idle is good. Where are these sensors you indicated, are they easy to get to?
Thanks
Jim

 

So If I am trying to apply a little common sense here, choosing between a bad throttle position sensor and a bad cam position sensor, it doesnt make sense to me to do the easier of the two(TPS). If the problem does not exist throughout all RPM ranges, or is not based on where I have the throttle, it would seem to me that the CPS is the more likely problem. I also have had the engine stall upon reaching the top of the hill at my house, it is almost as if as soon as I put a heavy load on the engine, ie., comming down to idle at the top of the hill, the engine stalls, but dropping to idle any other time is fine. Also none of these symptoms are occurring until the engine is warmed up and I have driven it for about 5 minutes. I am still puzzled on the speedometer not working either, is the speedo tied to either the TPS or CPS? Does anyone know of a forum that has mechanics willing to give good advice based on their experience? Just dont want to start buying $70 sensors for no reason. I did take the voltages at the center wire of the TPS, .700v at idle, and 1.4v at full throttle. This shows in the haynes manual as bad. Am I making any sense? Maybe time to take to the dealership
Thanks
Jim

 

Yup, TPS should be 3-4v at WOT. With voltage being lower than it should be, the PCM will think the throttle isn't open as far. The TPS is only part of the equation though (for fuel trim). RPM, MAP, and IAT are the primary inputs. If you're at a certain RPM, and the MAP and IAT (used to calculate airflow) are saying you're taking in a lot of air, but the TPS is "barely" open, then the PCM assumes a high engine load (like a lot of weight, or going up a hill), and will add extra fuel and pull timing (a lot). It the TPS voltage isn't actually constant, but flipping all over the place (which I'll bet it is if you tap on it), that fuel and timing is going to get varied, and you're engine will not respond well... Sad thing is, as long as it's in range, the CEL usually won't even throw a code.

Coolant temp has even less of an input to fuel trim. The sensor is a 2-part deal, with the accurate portion going to the PCM, and the "idiot" portion going to the cluster. Take a resistance reading across the bottom 2 terminals. With the engine at normal operating temperature (thermostat open, so 195F+), you should read about 700-1,000 ohms. At room temp, you should read about 7,000-13,000 ohms

Try a local salvage yard for sensors, they may let you have them for cheap (or free). The SOHC TPS is different than the DOHC TPS, rotated 90 degrees...

I'd bet on the crank sensor before the cam sensor. I've seen the cam sensor seals leak, and I've seen internal damage to a sensor from the cam (still can't figure out how...), but I haven't seen many fail. The crank sensor is much better know for failure.

If you know (or strongly suspect) a sensor is bad, replace it. It may not solve the problem, but it's not like throwing money out the window.