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Old 10-16-2011, 04:25 PM   #1
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Default Trying to solve hesitation/misfire under load at WOT...

I've got a 91 4dr civic with a D16A6. ECU is a PM6, the head was not milled, and it does have a new head gasket. Compression checks out perfect (190 across all cylinders). I originally converted it to MPFI and all the connections were done correctly; I never had any issues due to the swap. Then I dropped in a D16A6.

As of now, it idles perfect and revs quickly and smoothly beyond 4000 rpm's in nuetral. It drives great through all the gears as long as I engage the throttle steadily. But, once I floor it in any gear (hot or cold engine) below ~3500 rpm it just slowly builds speed until it hits 3500, then it takes off as it should. I know this is a misfire condition and it's unmistakable. It just hesitates to engage in the full throttle state and only when moving on the road. This is driving me nuts. Also, when my CRX was running, it would pull quickly and smoothly at full throttle on the road so I know this isn't psychological.

So far I've ruled out the TPS (tested perfect), O2 sensor (tested perfect), vacuum leaks (none), PCV (was working and cleaned), EACV (cleaned and working...entire intake manifold was cleaned and it only has probably 10K miles on it), new fuel filter, new distributor (had the stock one but no improvement switching to the new one), new spark plugs, new plug wires (new OEM ones), and I pulled off the catalytic converter but I could see through it pretty well and there was no black soot on the ceramic mesh or signs of damage.

I'm not sure what to check next? I had read that the misfire at low rpm's, under load, and at WOT is generally ignition related. I've only got the alternator left to replace on the ignition side (I've got 2 good working ones to try). If that doesn't fix it I'm going to check the fuel pressure, and if that is in spec I'll replace the injectors with ones from my other motor that I know worked.

Does anyone have any suggestions for what to look for or how to fix this? I've seen a lot of threads pertaining to this topic but most have no solution or it turns out to be plug wires or the distributor.

Any help/advice will be greatly appreciated. Thanks.

Last edited by crxsirg; 10-16-2011 at 05:25 PM.
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Old 10-16-2011, 04:28 PM   #2
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Default Re: Trying to solve hesitation/misfire under load at WOT...

You never said what ECU you were using?
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Old 10-16-2011, 05:14 PM   #3
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Default Re: Trying to solve hesitation/misfire under load at WOT...

Ah...pardon me. The ECU is a PM6. Everything is as stock as can be. No head milling and timing is perfect. I'll update the first post with this info, as well.
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Old 10-16-2011, 05:47 PM   #4
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Default Re: Trying to solve hesitation/misfire under load at WOT...

When you tested the TPS - did you SLOWLY go through the range looking for dead spots? (As in .1v at a time at the fastest?) Does your ECU have any codes? Possibility your ignitor or MAP is failing - though highly unlikely - but you would be throwing a code.
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Old 10-16-2011, 05:59 PM   #5
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Default Re: Trying to solve hesitation/misfire under load at WOT...

Quote:
Originally Posted by N3va3vaSatisfi3d View Post
When you tested the TPS - did you SLOWLY go through the range looking for dead spots? (As in .1v at a time at the fastest?) Does your ECU have any codes? Possibility your ignitor or MAP is failing - though highly unlikely - but you would be throwing a code.
No, I did not go 0.1V at a time (at least I don't remember being that exacting) but I thought I went reasonably slowly and it did increase steadily. However, I will test the TPS again and take your advice this time.

I'm showing no codes from the ECU. I actually replaced the MAP with the one from my other D16a6 car that didn't exhibit any hesitation.
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Old 10-16-2011, 06:32 PM   #6
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Default Re: Trying to solve hesitation/misfire under load at WOT...

cam timing
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Old 10-16-2011, 09:25 PM   #7
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Default Re: Trying to solve hesitation/misfire under load at WOT...

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cam timing
Could you elaborate?

I was very careful when I set the timing belt and nothing would indicate it jumped any teeth. Furthermore, I used a timing light to set the distributor correctly and I hardly had to move it from the middle of its allowable range of rotation to get the marks aligned.
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Old 10-16-2011, 09:47 PM   #8
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Default Re: Trying to solve hesitation/misfire under load at WOT...

is it bogging down (simply no power) or actually popping/bucking/jerking (running rough) when malfunctioning?
if it's bogging down I would suspect TPS or fuel, ignition or cam timing
if it's popping and bucking I would say you have an ignition breakdown.

I mentioned cam timing because it sounds like you've gone over most everything and your description sounds more like it's bogging down from what I'm reading. A tooth off on cam timing can run quite nice till you put a load on it, then it just bogs down like ****.

d16a6 (Si) cam gear marks should line up with lower mark on rear timing plastic. NOT with the horizontal marks/valve cover surface as is typical for most other configurations. See photo.

Click the image to open in full size.

what is your WOT TPS voltage?
also wouldn't hurt to know fuel pressure
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Old 10-17-2011, 07:37 AM   #9
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Default Re: Trying to solve hesitation/misfire under load at WOT...

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is it bogging down (simply no power) or actually popping/bucking/jerking (running rough) when malfunctioning?
if it's bogging down I would suspect TPS or fuel, ignition or cam timing
if it's popping and bucking I would say you have an ignition breakdown.

I mentioned cam timing because it sounds like you've gone over most everything and your description sounds more like it's bogging down from what I'm reading. A tooth off on cam timing can run quite nice till you put a load on it, then it just bogs down like ****.

d16a6 (Si) cam gear marks should line up with lower mark on rear timing plastic. NOT with the horizontal marks/valve cover surface as is typical for most other configurations. See photo.

Click the image to open in full size.

what is your WOT TPS voltage?
also wouldn't hurt to know fuel pressure
Thanks for the clarification. I set the timing correctly; I'm very sure of that. The car is definitely bogging. It seems to run smoothly but loses power on WOT (not completely though...it doesn't want to die out, it just accelerates with much less vigor than it should).

I will report back with my findings from testing the TPS more carefully.
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Old 10-17-2011, 08:22 AM   #10
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Default Re: Trying to solve hesitation/misfire under load at WOT...

Ok, so I just tested the TPS again and it's spot on. It increases nicely with no dead spots (I went at 0.1V increments) starting at 0.5V and stopping at 4.5V.
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Old 10-17-2011, 08:37 AM   #11
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Default Re: Trying to solve hesitation/misfire under load at WOT...

Check the fuel pressure... it can be enough to start and run the car but not enough to drive under load. Another possibility could be the fuel pressure regulator itself.
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Old 10-17-2011, 11:53 AM   #12
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Default Re: Trying to solve hesitation/misfire under load at WOT...

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Check the fuel pressure... it can be enough to start and run the car but not enough to drive under load. Another possibility could be the fuel pressure regulator itself.
Ok, I just checked the fuel pressure. I was getting 32psi at idle with the FPR vacuum hose connected and 40psi with it disconnected. The manual says it should be 40-50psi with the hose disconnected so I'm right at the bottom of the range but that should still be sufficient right? I feel like I'm running out of things to check/test. Thanks for all the help so far; hopefully I can get this resolved soon.
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Old 10-17-2011, 12:33 PM   #13
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Default Re: Trying to solve hesitation/misfire under load at WOT...

Now I am getting interested... What is the Vacuum at idle? Does the gauge fluctuate or flicker? Or is it stead and solid?

Sorry for all the one liners and such - but a step at a time.
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Old 10-17-2011, 12:46 PM   #14
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Default Re: Trying to solve hesitation/misfire under load at WOT...

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Now I am getting interested... What is the Vacuum at idle? Does the gauge fluctuate or flicker? Or is it stead and solid?

Sorry for all the one liners and such - but a step at a time.
Oh it's fine, I really appreciate the help.

I just used the vacuum line going to the FPR and it oscillated very quickly between 19.5 and 20.5 in Hg. I'm not sure if that's considered steady or not.
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Old 10-17-2011, 12:59 PM   #15
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Default Re: Trying to solve hesitation/misfire under load at WOT...

Ok when you rev it up - say 3k RPMs (or whatever) - does the needle constantly vibrate as it does at idle? How about under load?
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Old 10-17-2011, 01:05 PM   #16
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Default Re: Trying to solve hesitation/misfire under load at WOT...

Actually let me be a little more clear - is it "vibrating" OR is it "fluctuating" slowly (or slower than a vibration) moving between those numbers?
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Old 10-17-2011, 01:25 PM   #17
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Default Re: Trying to solve hesitation/misfire under load at WOT...

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Ok when you rev it up - say 3k RPMs (or whatever) - does the needle constantly vibrate as it does at idle? How about under load?
Ok...I think I didn't have the connections secure before because the vacuum was pretty much right at 20 in Hg. I also revved it up and the vacuum would drop and then resume.
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Old 10-17-2011, 04:24 PM   #18
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Default Re: Trying to solve hesitation/misfire under load at WOT...

Hmm... I thought we were on to something. So just to get a better understanding. What engine did you origionaly have in there that you converted to MPFI? What intake manifold did you use on that swap? Why did you change that swap out? What exactly did NOT get changed when you swapped in the new engine (not being specific but things like sensors and such)? Do you have any codes (should have asked earlier)? Is it the same tank of fuel as before the swap or did you recently get gas?

It's hard to diagnose something through a computer screen... with no visual or sound. Either way - there has to be something going on - so we can figure it out.
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Old 10-17-2011, 04:42 PM   #19
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Default Re: Trying to solve hesitation/misfire under load at WOT...

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Hmm... I thought we were on to something. So just to get a better understanding. What engine did you origionaly have in there that you converted to MPFI? What intake manifold did you use on that swap? Why did you change that swap out? What exactly did NOT get changed when you swapped in the new engine (not being specific but things like sensors and such)? Do you have any codes (should have asked earlier)? Is it the same tank of fuel as before the swap or did you recently get gas?

It's hard to diagnose something through a computer screen... with no visual or sound. Either way - there has to be something going on - so we can figure it out.
Originally it had a dpfi D15B2 with DX manual tranny. I did the mpfi conversion. The motor was progressively burning more oil so I decided to swap in a D16A6. I also had a spare Si transmission so that went in as well. The intake manifold and all the components on it were from a mpfi 4dr sedan I pulled from a junkyard. I cleaned every thing on it and assembled it with new gaskets before using it on the mpfi swap. When I swapped in the new/used D16A6 I just reused the intake manifold from the mpfi swap (fuel rail, injectors, sensors included) since the one on the D16 was dirty, the FPR was bent, and I didn't feel like cleaning another intake manifold (the one I used was still fairly clean; I checked it before bolting it on). I'd say only the charcoal canister, exhaust manifold, O2 sensor, and wire harness (aside from the mpfi wiring) were not changed from the dpfi engine.

I'm wondering now if I should replace the injectors...or maybe the alternator first. Would these things contribute to my problems? Without ECU codes I'm not sure what else to check. I think I've gone through all the troubleshooting guides in the manual.
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Old 10-17-2011, 04:51 PM   #20
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Default Re: Trying to solve hesitation/misfire under load at WOT...

The alternator shouldn't have anything to do with it. You can verify that by using your digital multi-meter and driving under load. Injectors are a possibility - though unlikely. It would most likely exhibit other signs. You could try to disconnect the FPR vacuum and see how it runs then.

The one question you didn't answer was the last one... Are you running the same tank of gas/recentley put gas in your vehicle?
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Old 10-17-2011, 05:04 PM   #21
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Default Re: Trying to solve hesitation/misfire under load at WOT...

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The alternator shouldn't have anything to do with it. You can verify that by using your digital multi-meter and driving under load. Injectors are a possibility - though unlikely. It would most likely exhibit other signs. You could try to disconnect the FPR vacuum and see how it runs then.

The one question you didn't answer was the last one... Are you running the same tank of gas/recentley put gas in your vehicle?
Heh...sorry about that, I completely overlooked it. I've put a little over 1K miles on it since the swap and it has had several fill-ups. I never really went WOT until recently because I was babying it to make sure everything was secure for a while. As of now it still has a full tank from the last fill-up.
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Old 10-18-2011, 04:26 AM   #22
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Default Re: Trying to solve hesitation/misfire under load at WOT...

Time for another slew of questions...

What did you gap your spark plugs at?
Were they the proper heat range?
When you did the wiring did you solder and heat shrink?
Have you started OHMing all the wires to the distributor/injectors?
I know they are new, but how about the spark plug wires?
Do you have a spare ECU to try?
When you set the timing did you jumper the service connector?
If you did, did the CEL come on?
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Old 10-18-2011, 09:13 AM   #23
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Default Re: Trying to solve hesitation/misfire under load at WOT...

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Originally Posted by N3va3vaSatisfi3d View Post
Time for another slew of questions...

What did you gap your spark plugs at?
Were they the proper heat range?
When you did the wiring did you solder and heat shrink?
Have you started OHMing all the wires to the distributor/injectors?
I know they are new, but how about the spark plug wires?
Do you have a spare ECU to try?
When you set the timing did you jumper the service connector?
If you did, did the CEL come on?
1.0 mm
yes
yes
no, but I'm double checking those now
maybe, I considered ohming them next
yes
yes
no
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Old 10-18-2011, 10:34 AM   #24
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Default Re: Trying to solve hesitation/misfire under load at WOT...

is there oil in your spark plug tubes?
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Old 10-18-2011, 11:10 AM   #25
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Default Re: Trying to solve hesitation/misfire under load at WOT...

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is there oil in your spark plug tubes?
Nope, I replaced the tube seals when I replaced the valve stem seals. The plugs look ok too; they have a grayish color.

I just ohmed out all the injector wires and they're good and where they're supposed to be.

Would doing the distributor timing with the timing light indicate whether or not the mechanical timing is off? I was able to get the distributor timing on the dot without having to really advance or retard it...I'm guessing that means everything is in time. I know I was careful with the mechanical timing but at this point I'm second (more like quadruple) guessing myself.
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