Idle problems

Mar 28th, 2003, 02:43 PM

Sorry this is so long but I have a lot of clues to give. Thank you in advance for any help.
GR

Have an 1985 (E16) Sentra 1.6 Non California 5-speed manual gasoline engine with A/C. I've been trying to track down an idle problem for a couple of years now though not too hard. When the weather gets cooler (below 60 degrees), the problem goes away. During warmer weather, I experience an idle speed, which fluctuates between 2000 to 2500 RPM's and normal (800-900 RPMs) and is not smooth. Often while idling more that 15 - 30 seconds, it will stumble and die. However, I can restart it immediately and will accelerate without stumbling or hesitation. One important point to note... While I'm rolling to a stop, the idle speed is always between 2000 and 2500. When I come to a stop, the idle returns to around 800-900 RPM's. Sometimes it will idle for the length of a red light, sometimes the idle will gradually get rougher until it dies unless I'm watching for it and give it gas to keep it running. I've also noticed that the RPM's are high when shifting gears. So, I think it's safe to say that whenever I'm rolling the idle is high, whenever I stop the idle returns to normal. I have an 85 Nissan Pulsar that I've traded computers with, and it still does the same thing. I given it a complete tune up, replace the oxygen sensor, replaced any vacuum hoses that were suspect, and even ran the computer diagnostics to see if I had a bad sensor. Other than the idle problem, the problem can't be too bad because the gas mileage is excellent and the performance is very good. Does anyone have any suggestions what it could be?

LittlebittaContact · Mar 29th, 2003, 12:10 AM

Have you scanned the ECU for any trouble codes? that will hopefully narrow it down.

Myetball · Mar 29th, 2003, 12:37 AM

Sounds like it may be a problem with your fast idle modulator. It's the big black thing on the right (drivers) side of carb. Visually inspect during fast idle, the plunger on the rear facing side should be extended and pressing on a lever which kicks up the idle.

If this is the culprit, it may be defective, or one of the mechanisms involved in controling the modulator is causing the problem.

Do you have a vacuum diagram? If not, I can scan one and send it to you.

Mar 30th, 2003, 02:31 PM

Thank you LittlebittaContact for your reply.

Yes, I've run the "ECU diagnostics" which in the manual states that it checks for proper oxygen sensor operation and can test the vacuum, water temp, Air temp, Mixture heater relay, air-fuel ration solenoid, richer solenoid, anti-dieseling, air- injection control, and vacuum control modulator. Due to the change in behavior of the problem due to changes in ambient temperature, I thought for sure it might be the air temp sensor but two different ECU computers reported that all was well.

And to Myetball,

The fast idle problem does not happen once I've come to a stop so visually inspecting the Fast Idle Plunger while the vehicle is rolling might be a problem but not totally out of the question.
I have the Sentra Service Manual and so I have the vacuum diagrams.

I really appreciate all taking the time to respond. I know that someone out there has seen this before and can tell me where to look. I know that this is a long shot but someone told me that the exhaust manifold and engine block must be ground properly so the oxygen sensor can relay the correct voltage reading to the ECU. I noticed that the ground strap from the radiator to the exhaust manifold shield is missing. I'm going to replace it today. Will keep you posted if it helps. Fourms are a great source of info!

Thanks,
Greg Ray

OZMAN · Apr 5th, 2003, 04:32 PM

TRY THE VCM VACUUM CONTROL MODULE,SAME PROBLEM I GET WHEN WORKING ON THEM.

Myetball · Apr 6th, 2003, 01:45 AM

Something you may want to check. The vacuum cut solenoid valve. It's located just behind the passenger side of the head, looks like two vacuum switches mounted right next to each other. Problem is, only the outer two nipples are used, the two inner ones need to be capped off. If these caps are old and cracked, they may be leaking causing the idle control module to disengage the reengage causing the engine to race.

I scanned the vacuum diagram from my Nissan shop manual, it's the only one that really shows all the connections. The Haynes and Chiltons show only the '84 connections for the B11. Scanned it at 300dpi so it won't be all fuzzy but is about 1mb. It's here:

http://www31.brinkster.com/myb11baby/vac.htm

Apr 16th, 2003, 06:05 PM

This might sound silly, but worked for me, try pulling your gas pedal out when its idling high. I had the pedal sticking on mine, worth a shot

7RIPP3R · Apr 16th, 2003, 10:15 PM

i dont know 86 but i think i'll try that too.. i've been having a problem with the engine racing and then idling after coming to a stop. I was thinking that maybe the pedal wire was getting stuck around in the engine bay by the carb. maybe needs some cleaning.

Apr 17th, 2003, 07:42 AM

I injected my throttle cable with STP Silicone Lubricant, maybe not the best thing in the world, but it worked.

gspot · Apr 17th, 2003, 12:03 PM

Sticky PCV valve????