View Full Version : S14 SR20 rough idle and throttle response issue.

04-03-2022, 09:13 PM
i have a 1990 s13 coupe with a notchtop s14 sr full swap done. relevant mods include fmic, upgraded rad, hks non recirc bov, full exhaust. bought the car already swapped and the idle has been rough since day one of ownership. now it has seemingly developed a throttle response issue that comes and goes at random.

the idle will jump around between 600 and 1k rpm, never dies though. always maintains idle, but its just rough. the car only had issue maintaining idle once after i started it after stalling it on a hill. :p

for the throttle issue, i can be cruising along normally and when i give throttle, theres no response for about a second or two. doesnt happen at all times, but will occasionally pop up. happened on one drive frequently but has only happened a handful of times since then.

the car runs rich but not excessively. pulls strong and has no issues under boost or while drifting/driving hard. on stock boost of 7psi.

the previous owner didnt build the car either, but thankfully he knows the person who did so im still learning about the car from who originally did the swap. the previous owner pointed to the tps as a potential cause of the idle issue.

before i dive into anything, any second opinions or other ideas would be greatly appreciated. :)

04-03-2022, 09:27 PM
what ecu? if stock did you check for codes, probably throwing a knock sensor code. if so you either need to replace it or you can put a resistor 1mhz 1ohm in the sub harness connector for the knock sensor to bypass it. I haven't run an knock sensor since I installed my motor.

04-03-2022, 11:16 PM
stock ecu. will definitely check for codes tomorrow.

bypassing the knock sensor may be a good idea.

any ideas on the idle issue? could just be minor vacuum leaks, will check for that as soon as i get the car to my families shop.

it hopefully isnt anything major, the car runs great besides the idle.

04-04-2022, 02:30 AM
There's a whole ass procedure to set idle on an sr which I cannot recall. but yes check all your hoses.

s13 @ fullboost
04-05-2022, 01:54 PM
You can play with the IACV adjustment located on the back of the intake, I believe counter clock wise will raise RPM & clock wise should lower.

The diagnostic moral here is that airflow sets the idle speed, the system should be pressure tested, make sure it is air tight i.e. you should be able to shut an engine off by covering the throttle body and similar ports (with your hand for example if you cover the TB/IACV the engine should die, if it kept running you have a leak somewhere)

fuel and timing influence/adjust idle speed, while airflow always sets the idle speed

How to adjust the idle speed on a high performance engine for maximum compliancy (very unlikely to suffer bearing damage due to low idle speeds, which may occur on engines with older ECU or old technology IACV type valves)
1. Get the engine warm, set idle Air fuel ratio, use 15* btdc on sr20det engines with a MAF sensor for best results, allow idle wander up to 22* btdc
2. Load the engine with headlights and electric fans, and any other devices you intend to run in the car frequently.
3. disable the IACV somehow; plug airflow holes that lead through any automatic idle control valves so no computer idle speed adjustments can occur for the next step
4. Engine warm and loaded up, set your idle speed using IACV manual adjustment (or throttle body base adjustment if the engine in question has no IACV) 10-60rpm below desired Idle speed.

5. Plug back in (re-enable) the IACV (or computer control) so the computer can use the valve to add that 10-60rpm back into the idle speed to achieve desired idle speed. If the ecu is OEM this is around 850rpm on sr20det. otherwise, units like the PFC have adjustable desired idle speed. If my desired idle speed is 900rpm for example, then my base idle speed with no computer control would be around 860rpm so the computer only needs to add a little bit of air to reach 900. Keeping the two settings (base idle and desired idle) very close will help stabilize the idle of the engine, especially if it has a long duration camshaft.

04-05-2022, 03:21 PM
You need to set base timing first. The cheapest software I can find: https://nisscan.com/NDSI/index.php?content=supportedcars

You can check codes, set base timing, and set idle speed

I would recirculate the bov to eliminate the variable. The spring could be set too weak and cause all kind of leaks

04-07-2022, 10:11 AM
Check the ECU for codes using the consult port. NDSI is a good one, and there's a few others. While you're looking, it's pretty easy to monitor the TPS voltage from closed to wide open in order to determine if that is an issue.

Check for vacuum leaks and verify base timing has been set, since those are the major causes for idle hunting. I'd guess one of the vacuum ports off the intake/throttle body isn't capped off, or has a cracked rubber line.

Once you get that sorted out and since you have a S14 SR, those actually use a wastegate solenoid to bump the "stock" boost to around 11psi. The wiring is there, you just need something like a MAC valve and connect it so that it bleeds air from the vacuum line going to the wastegate. Then really enjoy the full value of a stock-ish S14 engine, because running off base wastegate pressure sucks, feels so sluggish in comparison.

04-11-2022, 08:32 PM
Dead spot in the TPS sweep. Check TPS voltages.

07-10-2022, 01:54 PM
Did you ever solve the idle issue?

I agree with what others have said, but I did all that before finally realizing the issue was my alternator. Replaced it with a P2M alternator and it idles fine now.

Granted, mine was stumbling and dying half the time. I should have known right away when turning on stereo, AC and lights would cause it to stumble further.