Posessed 98 Sentra

[Ridge Racer]

I'm 'working' on a friend's 98 Sentra SE 2.0 liter. If I give it just a little gas, it will climb to about 2000 rpms, then suddenly drop about 300 or 400 rpms and then when I press the gas pedal down a little further, it will jump up about 700 rpms. It almost acts like it's dropping into a lower gear, but I know it's not.

I have checked out the EGR system and it's fine. The spark plugs and wires are new as is the cap and rotor and fuel filter.

It seems to be related more to the position of the gas pedal than the speed of the car. I can make it drop like that going 10 mph or at 60 mph.

I tested the voltage on the TPS and it checks out ok. Is there anything else that could go wrong with that or anything else that might cause such a confounding problem?

Thank you!

[NISTECH]

it sounds like a TPS rpoblem to me as well. How did it check out? what kinda voltages did you see just off throttle?

[Ridge Racer]

I can't remember what voltages I saw right off idle - but I remember the range from closed to WOT were within the correct range (.503V closed - increased to 4.09V at WOT) . I will backprobe that connector and see what happens near the low end. If it jumps and/or drops as I go up on the throttle, would that suggest that it's messed up?

[Ridge Racer]

Well, this morning on the way to work - I was driving the Sentra, about to get on the highway. I had to slow almost to a stop on the on ramp and then floored it to try to get up to speed before I got flattened by a semi.

When I punched the gas to the floor, the car took off and surged like crazy. The tach went up to about 5000, then down to 2000, up and down like that until I let off the gas. I punched it again - same thing. I had to just give it about 1/2 throttle and s l o w l y get up to speed. The onramp and this part of the highway are both on a fairly steep and long hill. I think that might have had a lot to do with it, because I haven't been able to repeat that on a flat road.

I will try to find some more steep long hills and see if I can make it do it again. Would this suggest anything in particular (timing or something)?

Thank you!

[NISTECH]

possible ign drop out. Could be the coil , could be the crank angle sensor in the distributor. When it "bogged" out did you hear any abnormal noises like popping from the intake or anything?

[NISTECH]

ahhh I thought this all sounded familier. You are on this car again trying to solve this very same problem. Do you still have cyl 1 misfire code? If I wouldnt have remembered this we would have gone through all the same stuff we did the last time.

[Ridge Racer]

Yup - still have the code. I wound up replacing the fuel injector after all of the other stuff and the code remains... (here's a link to the pages that have the stuff you and I went through: zerothread?id=121023 )

[Ridge Racer]

On the way home, going downhill this time, I was able to make the car surge again.

I drove around for awhile to see if I could pinpoint anything. It seems to make it to about 4200 rpms ok, but when it gets near 4500 - it starts to back off suddenly and if I leave the pedal to the floor, it will just surge then drop, surge then drop and run like crap until I let off the gas. It also seems to be getting terrible mileage.

I went out about 1 hour after I got home and tried to start the car - no dice. Sounded like it was having a really hard time turning the engine over. When my friend brought it over - he had to jump-start it to get it going. I took it to Autozone and had them test the battery and alternator - both checked out ok. I drove it around for 4 days without any trouble starting. I'm sure it's possible that their test was inaccurate.

Could a bad alternator cause any of the problems this car has?

Any ideas?
Modified by Ridge Racer at 7:13 PM 8/18/2005

[NISTECH]

no cause it would be a constant failure. It sounds like your losing a connection somewhere. With the car idleing wiggle the hell out of all the harnesses and see if it will cut out. Your either losing a connection or a component has a bad ground.

[Ridge Racer]

Well, I let it idle and wiggled every connector and/or harness I could get my hands on. Nothing I did seemed to make the engine slower, faster or any different. I know that there's a cause - somewhere, but it's really, really elusive!

Is there any kind of diagnostic test that would pinpoint the cause of anything that's going on with this machine of Satan?

[NISTECH]

Yea if its failing when the tests are done. But since it only does it when your driving and at higher rpm it would be kinda hard to jump out and test it. Have you put a coil on this thing yet?

[Ridge Racer]

"would be kinda hard to jump out and test it" LOL!

True! Nope - I have not tried a new coil yet. I will suggest that the owner look into that.

I don't remember the car doing that surging / lurching thing when I had it before. The last thing I did when I had it the last time was to change the #1 fuel injector. Is it possible that it could be causing that?

Thank you!

[NISTECH]

No it sounds like it is dropping out and kicking back in. The way you discribed it was exactly what happened to me on a v6 max when a coil was dropping out under accel at around 4000 rpms. If I held the throttle where it was at it would surge on and off till I was going nearly 20mph at which point the car started to accel and decel while the tac bounced from 1500 to 2000 rpm consistantly.

[Ridge Racer]

Is there a way to test a coil? If not - are they very expensive and/or difficult to replace?

Actually - I don't think that car has an ignition coil, does it?
Modified by Ridge Racer at 10:54 AM 8/24/2005

[NISTECH]

All gasoline powered cars have coils just depends on how many there are and how they are incorperated into the car. I thought your was remotly located but it could be inside the distributor in which case you would have to replace the distributor. That would be an expensive trial and air. There is a way to test a coil It requires you to check resistance of it. But this test will be useless as it would only show the failure when it is actually failing. There is another way to watch to see if its triggering but that would require an oscilliscope [expensive test equipment] You could rig it up to watch the trigger signal from the ECM. If that signal was still going out and the car wasnt firing then you would know the coil is bad.