SR20DET Ignition/Hesitation Problem

[TheBigS_42]

Hey guys, I started trying to solve this last Fall but didn't get anywhere so I'm gonna have another attempt now that the snow is gone.

I have a stock redtop in my 240 basic upgrades (FMIC, exhaust etc), nothing special (See the link in my sig for full details) However, the engine has been giving me some grief ever since I installed it. It runs pretty good while cold then after a few minutes of driving (sometimes 30 seconds, some times 10 minutes) it will start acting up and have major hesitation whenever I get into boost between about 2000-5000 rpm.

I have checked codes and originally had a knock sensor code. I replaced that and now I have the 55 code. All is good.

While working with a mechanic friend of mine, we discovered while having the car hooked up via OBD, that when the engine is running ok, the ignition timing appears to be pretty normal under load while boosting. However, when the engine starts acting up and hesitating, the timing is retarding to -6 degrees at worst and sometimes jumps all over the place when I get into any amount of boost. It will still idle fine but just surges and has very little power.

We suspected the CAS but after looking at it on an oscilloscope while driving it appeared to be sending a normal signal from what we could tell. We have also metered out the coil packs, injectors and coolant sensor and all appear to be well within specs according to the FSM. There is also plenty of fuel pressure when we checked (running a Walbro) and the regulator appears to be working fine.

I have out of ideas and any input would be much appreciated!

Thanks!

[camwhore]

monitor the knock voltage at the ECU (make sure you're using a DMM!) while driving. it should be ~2.5, but will fluctuate as it detects knocking.
the code is thrown when there's a break in the circuit and the computer sees 0 or 5V, NOT when it detects knock. the sensor and circuit can be working fine, but picking up knock and pulling timing.

when's the last time you pulled codes? did you redo the whole KS circuit? can't imagine you've been driving it much, how old is the gas? how do your plugs look?

[TheBigS_42]

Thanks for the reply! Its been parked all winter but when I was checking all that last year the gas was new. I just replaced the sensor at the end, the wires are original. I did the resistor trick at the plug though before and that worked so the wiring should be ok. The plugs look normal to me. I can try the DMM at the ecu when I plate the car in the coming days.

[TheBigS_42]

Update: I spent some time in the garage today with the FSM and a DMM. Heres what I came up with:

- No codes on the ECU. (Just 55 - OK)
- No boost leaks as far as I can tell (my gauge is on the FPR line and I'm getting 7lbs still so it should be fine)
- MAFS: voltages OK at harness
- ECTS: voltages OK at harness, resistances of sensor are in spec
- Knock Sensor: brand new, ground continuity at plug OK
- TPS: Resistances: 11k closed, 2k WOT. Voltages: 0.47v closed, 4.4v WOT
- O2 Sensor: in diag. mode II - cycles ~8-10 times in 10 seconds - OK. Voltage at harness = battery voltage
- All injector resistances are within spec (11.5Ohm)

This is really starting to bewilder me as it runs like a dream when cold but will eventually start having massive hesitation when getting into boost from 2-4k RPM. Doesn't matter if its half throttle or WOT, any amount of boost and it will hesitate. And it still is building boost, just not pulling.

Any more thoughts guys?

[Lobo240sx]

Check the mechanical timing and open up your ECU and look to see if it has an upgraded chip.

check the coupler on the actual throttle body. Make sure it is all the way back to the line. Just a little leak will cause weird hesitation problems.

[TheBigS_42]

Thanks for the reply. The timing has been set a few times to 15 deg BTDC a few times in an effort to resolve the problem but to no avail. I opened the ECU yesterday actually to check the same thing and it looks completely stock. I will double check the couplers again.

The weird thing is it runs fine when first started, leading me to believe it must be some sort of sensor or something when it gets into open loop or something temperature related? When we had it hooked up to a good reader at a friends work, the timing looked to be normal before the engine starts acting up when driving under load. However once it starts acting up, when I started getting into any amount of boost the timing was getting pulled way back I think. Kind of felt like some sort of limp mode, however once I got over 4000 rpm or so it would pull pretty strong again.

[TheBigS_42]

UPDATE: I think I have discovered the source of the problem! It seems to be stemming from the stupid knock sensor still. The ECU is not throwing any codes now (it used to throw 34 before I replaced the knock sensor), but given the symptoms, I had a suspicion it might be the cause. So I unplugged the sensor and shoved a 1 Meg resistor in there and went for a spin. I got the engine up to temperature and drove around for a long while to give it a chance to start acting up, but surprise, the hesitation was gone!

Now I know this might sound a bit ignorant or silly, but I really don't think that the engine is knocking, so is it possible that the wiring for the sensor or something might be good enough to stop it from throwing a code but wrong or damaged enough to cause problems? Could the new sensor (only got a few miles on it) be faulty but not throw a code? Is there some resistances or voltages I can check to test things? The FSM doesn't seem to be overly helpful on the matter other than continuity or what not. I have checked and I do have continuity from the harness plug to the ECU for the signal wire.

[TheBigS_42]

Tonight I tapped into the knock sensor signal with the sensor plugged back in. With the ignition on and engine off, and with engine at idle I get ~2.590V with very little fluctuation. When driving it is ~2.588V - 2.598V. This is before AND after the engine starts hesitating. My passenger had his eyes glued to the multimeter and didn't notice any spikes or changes in the reading before the engine started acting up.

Also of note, if I shut the car off for 10 seconds and restart it, it will run fine for a minute or 2 before it starts acting up again.

This is driving me crazy!

[camwhore]

Yah sure sounds like the old knock sensor; the hesitation, no hesitation >4k, goofy timing, code thrown. More common than the sensor itself is a short to ground in the sheilded wire (have been told its generally in the area of the fuel rail, after the KS subharness, I never bothered peeling it all back just to see.) This will also obviously still allow you continuity.

Has it come back at all since the first time? Perhaps your short is inconsistant and/or the circuit is getting noise but you rarely hit the 0/5v mark? My advice: play around with the suharness AND where it joins up with the rest of the harness. Monitor your voltages and see if you can get the code to throw then. Whether it does or doesn't, i'd still run a new sheilded wire from the subharness to the ECU, making sure to reconnect the grounds properly. If it works great, if not atleast you took care of a common problem and you can rule that out. Id also peel back the subharness and check that out.

[TheBigS_42]

Thanks for the reply. Interestingly enough, I've had to drive the car quite a bit to work and what not and the code has never come back since I replaced the knock sensor last year. I'm guessing the old sensor must have been bad, and while the new one might be good, if I have a ground problem like you mentioned, it won't really change anything by replacing the sensor. The continuity is good both ways as you mentioned but I didn't bother to peel back the covering to check the shielded wire. I think I will do that this weekend if I have time (it appears my brakes or something in the front end need urgent attention).

I'll be sure to update the progress.

Thanks!

[TheBigS_42]

So I'm back again, still with no good news

I've been getting desperate so I decided to run a completely new shielded coax style wire straight from the Knock Sensor through to the ECU. I grounded the shielding on the wire to the intake manifold and spliced the signal wire in close to the sensor and ECU, removing the old wire.

I got good continuity on both ends and the ground is connected well. Unfortunately 2-3 minutes into my test drive, sure enough, the hesitation is back. I checked the codes after and still get a 55, so the new wire must be connected properly at least. I am really at a loss here as to what could be wrong, it is driving me crazy!

Is it possible that the knock sensor I replaced is bad enough to cause false readings but not throw a code 34? Or is it just possible that my engine is knocking, but not loud enough that I can actually hear it?

Suggestions greatly appreciated!