Weather Dependent Issues

[Hoffman5982]

I've been having some weird issues for the past month or so. The motor is an s14 notchtop SR20DET that has been running perfectly for over a year. If it's cool out the car runs fine. If it's warm it stumbles constantly while cruising around 2k-3k. Above that it seems fine. Acceleration is smooth, but when it's in one of these "moods" it has a noticeable lack of power. It originally just started with this lack of power on warmer days and then escalated to the stumbling.

What I have checked/changed:

New TPS
New o2
New MAF
New ECU
New spark plugs
New IACV
New coolant temp sensor
New VTC solenoid

It's not a vaccum/boost leak. The only other things I can thing of are fuel related. I've been running a 300zx fuel pump for a hair over 3 years now. I need to get my fuel pressure tester back to see what that's doing. I have a spare oem FPR to throw on and test. Someone suggested that it could be vapor lock. Does that seem plausible? I do have my fuel lines tucked under the intake manifold with the filter(newer 300zx filter) directly behind the manifold. Could the heat from being close to the block be messing with it? Lastly, the ignitor. Has anyone heard of that causing an issue similar to this?

I don't think it's timing related as it was never messed with during the past year. I don't think it's wiring related as it's a Wiring Specialties pre made harness, and the two parts that I modified to slightly tuck have been looked over. I've looked everywhere I can and have already spent a decent amount replacing suspected parts. I'm completely baffled as to what it could be. Am I on the right track with suspecting it to be fuel related? Anyone who has come across these issues, I'd really appreciate some help. Thanks!

[fliprayzin240sx]

Id say check your timing since you havent done so. Base timing might be too advanced and making it noticeable when the engine is hot. Also, what spark plugs are you running and what is it gapped to?

[Hoffman5982]

Timing hasn't been touched in well over a year and its been fine til now. For plugs I was running bkr6e gapped at .032" and when I started I went to bkr7e's same gap and back to bkr6e's.

I'll put my timing light on it tomorrow to see what its doing

[jr_ss]

Go to .028 on the gap.

[Hoffman5982]

You think? I do have another new set of bkr7e's. Guess I'll throw them in tomorrow while I'm switching the FPR out.

[KiLLeR2001]

Sounds like fuel to me. Need to see what the pressure is at. Does your pump sound any different at all? Different tone etc?

edit: Stock gap is 0.036". Definitely go with BKR7E's, I had poor performance on the 6's.

[Hoffman5982]

Fuel pump doesn't sound any different. Its a z32 pump so the only time I really hear it is when the tank is low.

[fliprayzin240sx]

I was gonna say water temp sensor since its temperature related or a bad ECU but you've already replaced both of them.

[Hoffman5982]

It's only temp related to the weather. The car doesn't overheat and the problem exists when the car is cold and warmed up. But yeah, that was my first guess because originally it didn't seem to do when cold.

[OrangeVirus1]

The ignitor on my engine was bad, but the car wouldn't even start if it was hot. So probably not.
and when it would start it wouldn't stay running for more than 5 mins

[KiLLeR2001]

If your fuel pressure checks out fine, you may want to try another set of coilpacks. When they start going bad they act funky with increased heat.

[norcaldriftin]

try your cam angle sensor. I had the same issue on my 14 motor when it was hot. Replaced it and was fine. maybe use someone elses and see if that fixes the problem :)

[Mikester]

Could be as simple as condensation in your tank when the weather warms up... Make sure your fuel cap has a good seal and operates properly (S14 cap functions as a 2-way breather IIRC).

Besides coils, the coil pack subharness has been known to cause weird shit too. Although I hate to see someone spend a bunch of money & time on parts they don't need.

[jr_ss]

You never know it could be as much as corrosion on some electrical fittings as well. Do you still have you fuse box under the battery tray upside down? May want to check that out for condensation and or visible corrosion on connections.

[OrangeVirus1]

What if it's a bad knock sensor giving false readings when it's hot? who knows could happen

[Mikester]

You never know it could be as much as corrosion on some electrical fittings as well. Do you still have you fuse box under the battery tray upside down? May want to check that out for condensation and or visible corrosion on connections.

Point! I had a similar issue with my Toyota... Only when it would get really humid, the EFI relay connection under the fuse box would flake out... Causing everything from stumbles to flat-out not starting... Ya never know.

What if it's a bad knock sensor giving false readings when it's hot? who knows could happen

^^Yup- who knows. The one really frustrating thing about our motors is that there are like 1,000 different things that seem to cause the exact same symptoms. Lotsa fun, but not so fun to t-shoot sometimes.

[Hoffman5982]

Well I just got it back together. What I changed out was the fpr with a known good one, new z32 fuel filter(clean gas came out of the old filter, but it was probably due to be changed), and I put some heat shield around the fuel lines where they go under the intake manifold. I'll know tomorrow when I can get out and drive it if the issue is resolved.

In the mean time, I did notice that now when I start it it takes about an extra turn and a half to actually start. It starts nice and strong, so bogging and catching itself, and the car idles fine, but that's just something I noticed. I'm about to go back and put a new set of bkr7e's back in. I guess I'll let you all know tomorrow what the verdict is.

I really do appreciate the help guys. i'm glad I posted this here instead of the gen section.

Update:

Drove it today and it seems the stumbling issue is gone, and it's starting like normal again.

The bad news is that it feels like it's cutting power worse. There is no difference between acceleration at 0psi and 10psi boost. It seems to build and hold the boost just fine. What would cause this?

[Hoffman5982]

This is weird. Hooked my fuel pressure tester up right after the filter,and when I turn the key to acc it builds pressure, but as soon as it stops priming it drops back to zero. When I start the car it holts at 43-45 psi at idle. If I unplug the frp the idle jumps up a hair and the pressure raises to about 50. That all seems normal, but the dropping of pressure when it stops priming doesn't seem right. Does this sound like a pump issue?

[jr_ss]

That's normal... Atleast I remember my car doing the same and I didn't have any issues.

[Hoffman5982]

Well I guess I'm going to throw my other fpr back on and see if it goes back to doing what it did before. I'm also going to borrow a pump from a friend to see what changes, if anything. I have new intake manifold gaskets(since this is the third time I've removed the top plenum with the stock gaskets), and new valve cover/spark plug gaskets. For all I know the gasket between the plenum and runners could be leaking a little.

[Hoffman5982]

Drove it about 80 miles tonight and it cruises fine. So far, no sign of the stumble. It still has no power. It builds boost and pulls in lower rpms but starts breaking up above 3-3.5k. Vacuum is still strong so I don't think there's a problem there.

[jr_ss]

Sounds ignition related of your breaking up just getting into boost. Have you checked the ignitor and coils? It's not uncommon for them to go bad.

[Hoffman5982]

Well like I said I can get into boost until about 3.5k. I'm still trying to find someone with an ignitor I can borrow.

[KiLLeR2001]

I'm about 99% sure its one or more of your coilpacks. I remember a few years back I had this same issue.

http://zilvia.net/f/tech-talk/190641-misfiring-problem.html

[norcaldriftin]

I doubt its coil pack. I just had one go out and it would still boost hard but miss like crazy.

[Hoffman5982]

I'm about 99% sure its one or more of your coilpacks. I remember a few years back I had this same issue.

http://zilvia.net/f/tech-talk/190641-misfiring-problem.html


I'll start switching them around to see if there's any change. I should be a little more clear though, what I'm experiencing isn't so much a misfire. It literally feel like the power just drops off all of a sudden. The higher I go in rpm or the more gas I give it the less power I receive.

[jr_ss]

Knock sensor?

[Hoffman5982]

Wouldn't it be throwing a code if that was bad?

[blksylv]

i had a similiar issue when my cas went bad, car would run fine when could but would stumble and be hard to start when it was warm out.

[jr_ss]

Not necessarily... There is a method to bypass it to verify. I'm not sure what ohm resistor is needed, but I'll see if I can dig it up. You can try it to see if it stops "losing power" but I wouldn't use it as a fix all.

[Hoffman5982]

I have a friend with a blown motor. I'll grab his knock sensor.

[KiLLeR2001]

Wouldn't it be throwing a code if that was bad?

Are you pulling the code right after you experience the problem? It could be possible that the knock sensor is going bad when heated up, changing the resistance value. But if you check for codes on a cool motor, might not have enough variation to trigger an ECU flag.

[Hoffman5982]

Well I think it's resolved now. I put my old FPR on last night, and while I was at it I changed my VTC gear, valve cover gaskets, and spark plugs. Car drives perfectly now. I only drove it down the road, but at this point in time I have my power back and the stumbling issue is gone. I did find that the VTC gear wasn't the issue, so I'm eyeballing the solenoid.

As I said before, I really do appreciate everyone's help. Luckily it turned out to be something so simple, but if anyone digs this thread up in the future they'll have an idea what all they can check.

Moral of the story: Fucking fuel filters

[Hoffman5982]

Jk it started stumbling again today, although the power doesn't seem to be cut anymore