RPM spike when shifting into reverse

[zer0c123]

I have a 93 240sx 5sp

I got this weird rpm spike when i shifted into reverse today. I shifted into reverse somewhat hard and i heard this thump sound and feeling ( to me it sound like shifting from N to D in an automatic). Then my engine starting revving up to about 1500 rpm and my foot wasn't even on the gas pedal. This lasted for a few seconds then went back down to the usuall ~700rpm idling. This happens only if i shift hard into reverse.

I should also add that before this happened i was going down a slight incline in neutral. I can't remember if i was braking, but i felt my shifter rattle. Also saw my oil light blip on and off once, and the rpms went very low then back up again. All that happened real fast.

Nistech, if your reading this. Can this be related to my egr problem causing my car to have a rough ride? That problem was never fixed 100%.
Modified by zer0c123 at 10:02 PM 7/6/2005

[NISTECH]

Sounds like you have an electrical issue somewhere. I really dont think its related to your EGR issue. Start looking at all your grounds. make sure their clean and secure.

[zer0c123]

Is there any particular ground i should check? The ones that can see under my engine is the negative battery ground, and a ground wire that's attached to a screw on the passengers side next to the strut. I know there is probably another one for the transmission and the cat. Maybe i'll pickup an ohm meter or something and check all them and also jiggle all the wiring connected around and see what happens.

Heres some additional info that might help diagose the progblem. I noticed the weird rpm in stop and go traffic on a hill today. I put the car in 1st to move up a little, then clutch in and brake, but the rpm stayed at around 1500 rpm and slowly went back down to idle rpm. Being on the hill probably didn't matter, but it was where i first noticed the problem today.

Also I noticed that the rpm tended to stick sometimes around 1500 rpm. I noticed it a few times when i was coming to a stop. I just pushed in the clutch and brake, when the rpms started to come down, the needle kinda stuck at around 1500 rpm for a quick second. I'm pretty sure the rpm needle on my car is suppose to constantly, and not stop and go at different places.

Also this might be related to a problem that i posted here a few months ago zerothread?id=116104

but I think that problem went away after a tune-up and a timing adjustment. And, yea the lambo's are still there and i pass by that shop everyday. But i try not to stare too long or i'll rear end someone.

[NISTECH]

Try wiggling the harness near where it bends by the fire wall see if that will effect the idle.

[zer0c123]

I think i found the problem. I was wiggling just about everything around the engine bay, until i wiggled the vaccum hose that connects from the front of the engine to my aem intake (refer to the images below.

Note: I just pulled this image from another thread.

I wiggled that hose and the engine stuttered, then slowly reved up to 1500rpm. Since you said it sounded like an electrical problem, i think it's the plug that is wrapped around the hose that is causing the rpm spike. I test it out a few times , but stopped because my whole garage started smelling like rotten egg.


Modified by zer0c123 at 11:29 PM 7/12/2005

[NISTECH]

Check the connection but it may be the boot the hose is attached to is split. The smell you got is because the cat started over heating due to the lean condition. Really look over the hose for splits or cracks.

[zer0c123]

Before i started the car today i felt around the plug to make sure it was fastened correctly and felt around the hose. Then started the car, but the car felt really rough with the rpms spiking up and down.

I turned off the car and removed the hose. There doesn't seem to be any cracks or holes on it and also the boot that the hose slides into didn't seem to be damaged. The only thing that i could see on the hose was the imprints left by the metal fastners that were inlucded with my intake. The ones you have to screw on to tighten.

I restarted the car and the idle was not like before, but now the car seemed to rev coninuously. I observed the rpm meter, it was ~900-1100, it was slightly reving in some continous loop. The rpm was high because it was a cold start. While this was happening I messed with that wire circled in green above (might have to zoom in to see it). Then the car stuttered and almost stalled. The reving loop doesn't really go away after some driving. Now the idle is somewhat unstable at times.

Modified by zer0c123 at 11:57 PM 7/12/2005
Modified by zer0c123 at 12:13 AM 7/13/2005

[NISTECH]

looks like you might have circles the coolant temp sensor. unplug it and check its connection for corrosion. Clean and tighten the connection if you can. Then recheck it.

[zer0c123]

I thought the coolant temperature sensor looks like a plug with a colored top? I wiggled the wire at the top end of a snap on connector that looks the same as the TPS sensor but only this one is a smaller size. The wire follows up into that black rail that is in front of the fuel injectors. Heres a better look

[NISTECH]

If your talking about the conector you have the red moark on in this picture that is your TPS connector.

[zer0c123]

lol, i always thought the TPS connector was one on the top left. So the wire i messed with, was the one coming into the TPS connector from the right.

I unplugged the connector and inspected the inside. There didn't seem to be any signs of damage, but I could see some dust in there. After i reattached the connecter and i tried pushing in the top and bottom wires of the connector.

Now when i wiggle that same wire or the whole TPS connector itself, nothing happens. However, my rpms still do a continuous loop of slight rpm spikes. Looks like I may have to replace it?

[NISTECH]

no If your idle is jumping up and down like that try adjusting your tps a bit it might be on the edge of its hard idle contacts. The voltage on the center wire of your tps should be .5volts or slightly under that value. You do the test with the connector plugged in and back probe the signal wire.

[zer0c123]

Can't believe this happened. When i was setting up the TPS sensor to test the voltage, the black wire (ground i guess) snapped. It snapped off from the connector that connects to the TPS connector. There wasn't enough slack left to use electrical tape to patch it.

I'm thinking that i can get the connector of another 240 from the junkyard. I saw SOHC model there earlier, hopefully it uses the same connector and has the same # of wires.

[zer0c123]

ok, i finally got the connector from another engine harnessI just replaced the connector by just snipping it off and electrical taping it onto my engine harness.I backprobed tps voltage with the ignition just in ON position, not started and got the these results. Left wire: 5.12VMiddle wire: 0.46Vright ground

I also measured just the connector that connects to the TPS connector with the ignition key off i think (dont remember). Measured it because i didn't know what i was suppose to backprobe until later.left wire: 5.12Vmiddle wire: 4.58Vright ground

The car seems to idle pretty good now. I guess the weak ground wire was causing all the problems. I basically had to cut, splice and tape wires together to get the connector on. Is it ok to leave the wires electrical taped together like that? I'm afraid that since the engine bay produces alot of heat, it might melt the tape or something. I also noticed a ground wire in the back of the engine that was snapped off from a plastic connector screwed into the firewall. The wire was darkgreen and looked like a shoelace to me. It was getting late, so i just used electrical tape and taped the wire to the firewall as a temporary fix.

[NISTECH]

You need to soldier the wire connections to the tps and use heat shrinkto seal it or it will develop corrosion and cause drivability problems. as far as the braided ground ,just run a screw through it where is originally bolted to the fire wall.