SR ignition timing hesitation problem

[TheBigS_42]

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 the ignition timing appears to be pretty normal under load while boosting when the engine is running ok. 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.

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 and the regulator appears to be working fine.

We are running out of ideas and any input would be much appreciated!

Thanks!

[compactfean]

It sounds like your ecu is going into a safe mode. How many counts of knock do you have during driving? The knock sensor, if doing its job, can detect an issue and cause this.

[TheBigS_42]

As far as I know there are none. We tried unplugging the knock sensor temporarily while it was acting up and the problem persisted.

[compactfean]

Unplugging it will do the same thing. unbolt it and put it in,a sock and ziptie it away from everything.

[TheBigS_42]

Hmm, ok I might have to try that. If you or anyone has any other ideas before I do that I might wait a bit. I just replaced it so the memories of how annoying it is to reach (had tto take the starter off to fit my arms in there) are still very fresh haha.

[compactfean]

If you can borrow one from someone just to plug in away from the motor just for testing? That way you don't have to magically have chinese fingers! Lol.

[TheBigS_42]

I should have remembered, I have done the resistor trick where you shove a 1 Meg resistor in the plug to the ecu for the knock sensor, to trick it into thinking all is well. The engine still acted up with this in place. I assume this would be the same as if I had another knock sensor connected for the purpose of this test?

[the converted]

When you have it hooked up what are you seeing for coolant temps? That's about all I can think of.

[TheBigS_42]

It peaks at about 176F (80C) when warm.

[Speedracer5767]

I have the exact same problem! I also have tested everything. New sparkknock plugs gappedin at .030. Checked timing and everything seems to be right. I even replaced my knock sensor with no avail! I do have 1 question? I tried checking my voltages on the mass air flow and I'm not sure if I got an accurate reading or not but if I were to get accurate readings they should be as follows--> 12v supply, ground and 5v signal reference. Right? I have been pulling my hair out because of this problem. It feels amazing when its cold and it runs so strong and then soon as it warms up it's like it falls on its face under boost.

[Speedracer5767]

BUMP for any opinions?

[TheBigS_42]

Ill have to double check my maf voltages but I'm pretty sure they checked out when I checked it last. I'm not sure what else to do. This problem is driving me crazy.

[Speedracer5767]

I feel you man. It sucks because it bogs down so bad under full throttle. But while its bogging down if I let out of the throttle a little it seems to help it a bit I guess just because I'm not trying to push too much boost. My wideband is reading around 14 when it does this. Its pegs rich first and then it starts to lean back out. The computer must be pulling timing. What's crazy is that everything is spot on with timing, and it runs so well before it warms up. I'm going to probably buy another coolant temp sensor and see if that helps. I don't know what else to do. And my computer does not have an led so I can't even check codes. I am probably going to have to get a computer with NISTUNE so I can datalog and check codes.

[compactfean]

You can get a consult cable from e-mance and read all that on the stock ecu. As long as you have the ecu - dash plug wired correctly you can use the stock obd1 port. If its not wired up you can snip your obd1 plug that's already in your car and just wire it directly to your sr harness. No need for a completely different ecu.

[Speedracer5767]

I wired in my factory consult port and now I can read my datastream. Now I just got to plug in my shop scan tool and drive my car when its acting up.

[TheBigS_42]

Its been a while but I've finally got some time to work on things. 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?

[compactfean]

Fire some reason fuel system comes to mind. Fuel pressure? New fuel pump or filter?

[TheBigS_42]

Thanks for the reply. I thought it might be fuel too, I forgot to mention we checked the pressure last fall and it was good both at idle and under load (above spec if anything since the car has a Walbro 255). I replaced the fuel filter recently in hope that would help things with a 300ZX one but it made no difference.

[TheBigS_42]

bump

[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.

[TheBigS_42]

Also, 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!

[compactfean]

How tight did you torque the knock sensor? Usually I under torque my knock sensors because sometimes id its to tight, you can pick up unwanted noise...... Especially with solid lifters!!! Lol

[TheBigS_42]

Interesting. I had a hell of a time getting at it so I didn't use my torque wrench to put it on. I'm guessing since it took the strength of Thor to get the old one off, I probably over-tightened the new one. When I have some time this week I'll climb under there again and redo it and see if that changes anything.

[TheBigS_42]

Well it was worth a shot! Unfortunately I took it off, checked everything with a DMM, cleaned both surfaces and put it back on and nothing has changed. Still starts to hesitate a few minutes into driving every time without fail. I'm starting to think it must be a ground problem or something. The sensor must be good because it isn't throwing a code like the last one was, there is continuity on both parts of the KS harness, but maybe theres a ground problem, which wouldn't necessarily affect continuity readings. Should I run a new shielded wire to the ECU from the KS harness and replace the shielded wire on the KS harness itself?

[TheBigS_42]

Gah, I'm back again, still with no good news.

So I've been getting desperate so I decided to run a completely new shielded coax style cable 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 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!

[compactfean]

Have you preformed the test for the cts according to the fsm? I've had coolant temp sensors be out of range and not throw a code.

[TheBigS_42]

I did check it a few weeks back and got the required 10v, 0v, and 5v I think that was required in the harness plug in whatever the procedure was (I just have the results written down). And the resistances all were in the required range. Unless I didn't measure something or missed something I think the CTS is fine, but I could be wrong haha