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Hesitation/Intake Backfire Under Light Throttle... Running Out of Ideas


Cannonball89

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Hi guys, it's been a while since I've been on here. I have been busy with work and school, but I have been enjoying my turbo swap 240z for the past several years with Megasquirt with no issues whatsoever until recently. I know this thread could probably go under the Troubleshooting forum, but I since my car is running MS3 I thought it fit better here. 

 

My problem started about a month ago. I was driving it with my buddy on a hot day and it was running great, then I stopped to drop him off and turned the engine off. When I restarted it about 2 minutes later, I noticed as soon as I pulled out that it was hesitating, and if I pushed the throttle down a little further it would violently buck and pop out of the intake. Once I managed to get it up to speed, it would cruise OK, but the exhaust note sounded a little funny. However going up hills or accelerating it would continue to buck and backfire. I managed to get it home and hoped that it was some heat related issue, and that if I let her cool down over night it would be fine the next day, but that turned out to not be the case. 

 

I know that this issue is most likely related to either a lean condition or a spark issue, and I have troubleshooted and attempted to diagnose it with no success so far, I am really hoping to come up with that "Ah Ha!" moment, but it just isn't happening so I thought I'd ask you guys. The strange thing is that my datalogs, which I will attach do not show AFRs going lean when it begins to hesitate and backfire, which makes me lean towards it being a spark issue or perhaps an issue isolated to one cylinder.

 

On a side note, I installed an EBC about two weeks before this started happening, but I can't imagine that the EBC is causing this issue since I am not even under boost when the problem is occuring. I also put in the MSA Fuse box to replace the old melted stock one, but everything related to the engine and MS is on separate circuits with a dedicated fuse box.

 

Here is a list of what I have tried so far:

 

1. Cleaned corrosion off of the inside of the cap and rotor.

2. Installed new NGK BR8ES plugs, properly gapped at .025

3. Checked impedance on spark plug wire, and checked for arcing.

4. Checked voltage at coil, fuel pumps, and at MS, and found a solid 13-14 volts at idle.

5. Checked compression at every cylinder, no abnormalities, a consistent 145 PSI in all 6.

6. Visually inspected valvetrain for damage, looked OK.

7. Verified that spark timing in MS agreed with what the timing light said.

8. Blew compressed air through all fuel lines.

9. Replaced both of my fuel filters.

10. Drained the tank and put fresh fuel in. 

11. Visually inspected entire wiring harness for loose connections or worn off insulation. 

 

So far I haven't found any problems and it is getting frustrating. My next step probably will be to ship the MS back to DIY and have them check it out, and possible send my injectors off to be tested and possibly rebuilt. I may send the coil back to MSD to have that tested too.

 

I am hoping some of you can offer some more suggestions.  

 

A datalog of me driving around my neighborhood is attached in a zip folder. The hesitation occurs basically whenever I am on the throttle. I can't find anything unusual, but hopefully one of you guys can.

 

Thanks

 

Datalog.zip

 

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I can not open your file right now, before you get too far double check for vacuum leaks or broken hoses. Something had to change to cause the sudden change, it sounds like spark to me, check for interference. If your vr sensor is picking up interference then the first spark with be dead on but the other 5 cylinders will be off. I hooked my vr sensor up backwards one day and it threw me for a loop cyl #1 was dead on, but all of the others ones had been off by quite a bit causing some intake popping. 

 

Good Luck,

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I am using an MSD Blaster 2 coil and I am using a stock 280ZX Turbo dizzy and CAS. So basically it just has six notches that the computer counts. I am not running sequential injection, just alternating bank fire so that set up is adequate. I don't think that any noise is getting into the signal, the trigger log looked pretty solid when I checked it. And it doesn't seem to correspond to any specific cells in my maps, it just seems to be whenever the engine is under load. I haven't noticed any loose vacuum lines either.

 

If the distributor drive gear had somehow jumped out of place, my timing should be out of whack when I check it with a timing light, correct? I am also wondering if it is possible that the Timing Chain may have somehow jumped a tooth, but that seems unlikely too.

Edited by Cannonball89
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 Is the noise filter enabled. I have MS2. When I disabled it I had ignition issues above 3000rpm. I realize you have MS3 and said you didn't think you had noise issues. I didn't think I did either until I disabled it. :)

Edited by ozzzzz
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pop out of the intake.

I've seen several small block chevy engines do this after wiping a cam lobe.  Might check the mechanicals since the electrical and fuel look right.

 

I've seen it described also, on this forum that the press-fit distributor shaft can spin.  Timing  verification is a good idea. 

 

Checking for a tooth jump on the cam chain is easy enough, using the notch and groove, and the zero timing mark.

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Done a ohm test on the coil. Sounds like spark isn't holding up under load.

Should be some specs to compare to for that MSD Blaster coil.

 

Ok, according to MSD's website, the specs for the Blaster 2, model 8200 are:

 

Primary Resistance: 0.7 OHMs

Secondary Resistance: 4.5K OHMs

 

According to my multimeter, my coil registers at:

 

Primary Resistance: 1.3 OHMs

Secondary Resistance: 4.8K OHMs

 

So, I think it is pretty safe to say that we have found a winner? Primary winding resistance being almost double what it should be is most likely responsible for a weak spark that makes the car fall on its face I would say, correct?

 

Thanks all you guys, sometimes it just takes more than one brain to think through all the possibilities.

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Whats the resistance of your multimeter leads?

Say what? You should always check for 0 ohms before measuring. Never heard of testing or measuring the tester leads. Then you better test the tester that tested your test leads:0

 

You have a stock coil to try out as suggested?

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