Misfire at idle, related to '75 intake manifold?

[Guest bastaad525]

Just thinking out loud, looking for possible causes of what I think may be misfire.

 

 

Facts that may or may not be relevant : I run an MSD 6A ignition, 6 mos old NGK plug wires, NGK plugs, stock heat range, overgapped to .045-.050, and an MSD blaster 2 coil, which is about 3 years old and kinda beat up looking (this is one suspect). Last I checked, compression was good and even at 120-125 psi on all cylinders (my gauge is a cheapy gauge and reads way lower than my friends snap-on gauge, verified on his car not on mine so I dont know my numbers). Fuel injectors are stock turbo injectors, and while they are old (and suspect) whenever I pull the plugs they are all just about the same, which tells me no injector is running much different than the others... I've also run all kinds of fuel cleaner thru them... evetually I'll remove them and have them profesionally cleaned and flowchecked. Fuel filter is about 6 mos old, and my fuel tank was cleaned and recoated about a year ago so not worried about dirty fuel or filter. Valves were adjusted about 2 mos ago and are very quiet.

 

Now to the specific question. When I had the head rebuilt a while back I decided to switch over to the '75 280z manifold, which is missing most of the stuff that clutters up the ZXT manifold. Stuff like EGR, VCV, AAC, cold start injector, cold start valve, are all missing. The only things attached to my manifold are vacuum ports, for FPR, boost gauge, brake booster, and BOV. That's it. As far as I know I have no vacuum leaks, and have checked for them many many times, so pretty confident in that.

 

Now what I want to know, does it make sense for any of this missing stuff to possibly be the cause of the occasional popping and slightly rough running at idle and low RPM? It bears mentioning that I run with my TPS and O2 sensor disconnected as well. With the TPS connected the idle seems rougher, but the popping lessens... I leave it disconnected because when I drive with it I get this annoying off idle stumble (this isn't just me, Jersey mentioned having the exact same problem). I tried three different TPS and got the same result. The O2, if connected, causes the motor to run lumpier overall AND go pop-pop-pop more... just overall seems to run crappy with it (again, this isn't just me, a few guys on here have reported the same issue). This was even after installing a new, factory nissan O2.

 

One other thing, I was able to tune out the pop-pop-popping a bit, by tightening up the air flow meter spring, leaning out the fuel mixture slightly. I only tightened it by three gear teeth. As far as I know it was set to stock position before, but ran pretty rich on the dyno up to about 4500 rpm, which is why I initially leaned it out... the less popping thing was just a good side effect. However, if I lean it out any further, idle gets rougher, and the popping comes back in force.

 

 

Any ideas? Should I just attribute the rough idle to the missing manifold components and forget about it? otherwise the motor (usually) seems to run great. Should I keep investigating the EFI (done a LOT of looking at that)? Any other ideas? There was a time when this motor would run so smoothly that you almost wouldn't know if it was running... would like to get back to that if possible.

[240hoke]

Yup i'd say it is the manifold. Mine does the exact same thing. There is a reason for all that stuff on there you know. BUt then again ive never run mine with all that crap one it, so i dont really know.

 

-Austin

[technicalninja]

As long as no vacuum leaks are present any Z manifold will work fine with out misfire. Some far more likely suspects are corrosion in wiring harness (connectors) and injector patterns.

Injector patterns can be checked by removing injectors and rail system from intake (leaving fuel hoses attached), pressurize fuel system, and apply a 9 v battery across each injector terminal (remove injector harness from injector). You should have a nice clean upside down ice cream cone shape. Check each injector multiple times as when an injector is having problems it doesn't have problems all the time.

Be careful as you will be atomizing fuel into the atmosphere (very flammable). I use a 2 L bottle that I have cut thr very top out of to catch the fuel.

This test is done with the key out of the ignition and the F.P. relay jumpered.

A bad pattern will have breaks in the cone shape and sometimes it will spray in a solid stream (called streaming). fuel that is not completely atomized will not burn until it reaches the exhaust manifold causing your pop-pop.

The TPS puts EFI brain into idle mode (different FI map) and should be used. I do not think clean idle can be had with out it. This goes the same for the 02 sensor. With out the 02 installed the brain is receiving signal (actually lack of signal) that makes it think car is very lean causing it to richen mixture at all times.

Install and adjust TPS, add 02 back into system, adjust afm to solve off idle flat spot.

A test you can do to simulate a good 02 sensor is to grab o2 sensor harness making skin to metal contact with the sensor wire and with your other hand grab positive terminal of battery. Try this while idling and see if idle smooths out.

Although these cars can run with out some of their systems Nissan would not have spent the money to instal if they were not nec. Disconnecting things is not the "proper way" to make modifications. tricking signals or remapping fuel curves is a better choice. Stand alone systems are the best choice IMO.

I bet you injector patterns will be your culprit. Look closely at purchasing "rebuilt" injector. My parts supply house allowed me to unbox some Beck Arnley "rebuilts" allong with some Beck Arnley "new" injectors. They were identical including the silk screened part # on their sides. Cost me $30 each when individual cleaning costs me $22. No brainer and they have worked perfectly for me.

Hope this helps

Rick

[turbobluestreak]

well bastaad525

Stuff like EGR, VCV, AAC, cold start injector, cold start valve, are all missing.

 

There is no cold start injector on a turbo intake, but on that intake there is a provision for it on that 280z intake. I have one intake reads Japan N42 and I'm getting ready to install it with my 60mm t/b. but I've allready removed the EGR, AAC, FICD, Air regulator, and CAT. I think theres a small stumble at idle but my boost gauge is reading 17 inches at idle

 

Best solution convert to 300zx ecu and remove all that junk.

 

 

 

tbs

[Sean73]

1) Without the O2 sensor, you're engine cannot ever go into closed loop mode at cruise and part throttle conditions. So, the ECU is probably defaulting to some type of limp mode or default program. So you have an A/F gauge? Although A/F gauges aren't accurate enough for tuning, one thing they do tell you is when your ECU is in closed/open loop. This information can be handy for diagnostic work.

 

2) Without the TPS connected, the engine is unaware that it's idling, and therefore, the ECU is probably reacting similar to #1, and explains why your not idling well.

 

3) You could be running too rich because the ECU is seeing start signal all the time. I've heard about this with many turbo swaps, where the start signal wire is hooked up to ignition positive, causing the the ECU to run rich all the time.

 

Ok, you've said that the engine idles even worse with the O2 and TPS connected. Have you checked your CHTS? A bad CHTS could be causing an overly rich condition at idle, and then with #2 or #3, it gets even more rich, causing the engine to idle worse.

 

First thing I would do is check the CHTS, and see if it's within spec. If so, hook it up. Check the TPS with an Ohm meter, and hook it back up. Check the O2 sensor for fouling, then hook it back up. Check wiring for proper start signal. Check all of your groung wires too. All of the tune-up stuff you've done is good, but with sensors disconnected, the ECU really can't, and won't do it's job. Good luck.

 

Sean

73 240Z, L28ET, T3/T04E, NPR IC, Z31 ECCS

[Guest bastaad525]

Okay... some more details... I've been chasing this and other problems in the EFI for a LONG time now, and have actually done a lot of what's mentioned here already:

 

 

I've tried three different TPS's (used)... and tested and adjusted each one using a volt meter to go open with just a light touch on the throttle... idle is set at about 800 rpm, tps goes open at around 825. All three of them seemed to be working fine, yet all gave that off idle stumble, and tuning the AFM spring tension with TPS connected did NOT affect this at all. Again, others have noticed this stumble.

 

The head temp sensor has also been replaced twice, admittedly with used units, though the last one I installed I did test with a pot of water, a meat thermometer, and a voltmeter, and it was within spec.

 

The old O2 sensor actually wasn't that fouled when I pulled it out. Replaced it with a factory new nissan O2 and the car ran exactly the same when connected. I did connect it and the TPS at the same time so everything was connected as it was supposed to be (minus the missing stuff on the manifold), and I still had my stumble, and I still had a VERY rough idle. Unplug the O2 and about 5 seconds later the idle cleans up nicely, but there is STILL some popping, and still stumbles off idle. Unplug the TPS and the idle jumps/stumbles for a second (just like when the throttle is first pressed and TPS would normally go open) and then just reverts to normal, I think it idles just a tad bit lower, maybe 50 rpm, if I remember correctly, but otherwise the same, no more or less rough, no more or less popping from the exhaust with TPS connected or not. Just the off idle stumble which goes away when it's disconnected. I know of at least three other guys on here who claim their Z's run better with the O2 disconnected, and all three first tried installing new sensors and this didn't solve the problem. Additionally, I've also tried running the O2 w/o the TPS, and same deal, very rough idling with O2 connected.

 

As far as all the EFI connections, I have cleaned every connector SEVERAL time, including all the connections at the ECU, AFM, HTS, TPS, CAS, and all injectors.

 

All my grounds (five grounds off of the wiring harness, two thicker and three thinner ones) are connect either to the intake manifold, or to the body of the car. The connections are clean, the paint sanded off of the area where they are attached to the body. I've no doubt they are all okay.

 

I know the injectors are still suspect... I have tried several dump-in fuel system cleaners, including Techron, and the Neo fuel system cleaner (this stuff supposedly works miracles, according to the friends I have who've used it!). None of them seemed to affect it. For what it's worth, I believe they are aftermarket injectors. I didn't know Beck Arnley injectors were so cheap... but I somehow think they wouldn't be so cheap for me cleaning and flow checking would probably be the cheaper option... I may just take the opportunity to switch up to some 370cc injs.

 

I'd bet my money on injectors first, if not then wiring harness, maybe corrosion I can't see or get to, or just worn wires, if not then some calibration is off and yeah the TPS and O2 need to be reconnected and then something readjusted. But I will say this again... there WAS a time, believe it or not, when the idle used to be much smoother... almost new car smooth... when there were times I'd stop at a red light and think the motor may have died. This was with the TPS and O2 disconnected... as I've never left them connected for very long at one time since installing this engine. Sounds rediculous but I swear it is true...

[Sean73]

Did you check the start signal wire?

[Guest bastaad525]

Did you check the start signal wire?

 

 

I looked into this a while ago going off of one of Jersey's threads, as he personally went thru that issue, what symptoms he was having and how to take care of it. To be honest, I never found which wire I was looking for or what I was checking for after like an hour of looking at the ECU wires. But I will say, I dont have any of the symptoms he or others described as being associated with the start signal wire problem. I mean he was saying how it was running super rich... I definately am not running rich like that during around town cruising, or in general I believe. The motor DOES run really rich under WOT boost, but MANY other ZT's I've seen, not just swapped 240zt's, but stock 280zxt's as well, run pretty rich under boost from 3000-4500rpm... seems to be more a function of how the stock EFI is set up. But otherwise I wouldnt' say I'm running rich at all, my plugs are always a nice tan color when I pull them out... also, throttle response is pretty good, doesn't react like a super rich motor, and it accelerates well except for a bit of turbo lag caused I assume by my very restrictive 2.5" pressbent exhaust and crappy glasspack.

 

However I AM pretty sure that I'm running a bit too rich at idle only, with the TPS disconnected the way it is. If I let the car idle a long time the plugs do become sooty black... but they never load up so badly as to cause the motor to run badly like my SU'd motor did. I've let the turbo motor idle for an hour before and it still ran fine... pulled the plugs and they were sooty but not overly so.

 

I will take some time in the next couple days, hunt up that old thread of Jersey's, try to figure out exactly what I"m looking for again and try to check and be 100% sure the start wire is hooked up correctly. If I can't find it I'll post a new thread.

[cygnusx1]

I am in the same boat right now and by playing with the idle mixture I can get the pop-pop-bump-stumble out of the idle. As a matter of fact, I could probably stand a nickel on edge on the motor when I get the idle mixture right. To do it I need to turn the wheel in the AFM one way or the other until it gets the sweet spot. The problem is when its at that sweet spot at idle it runs wrong under power I have ALL of the manifold "controls" connected and working and I still get a lumpy idle when the driving mixture is set to run good (butt dyno). I find either, or, but never both. My mind is spinning with "rigged" workarounds for this problem. If I get a solution I will let everyone know. I personally don't think its the manifold thingy's since we both have very similar idle problems and two different manifolds.

 

BTW, start signal is wired properly on mine.

 

I am sure you saw this post too...

http://hybridz.org/nuke/index.php?name=PNphpBB2&file=viewtopic&t=34554