l28ET Z31 ECU/MAF leans out 3500 RPM on the dot, Any driving load.

[SDgoods]

Well I thought I knew the system quite well but I cant figure out this problem... I redid some intercooler piping and went as far as using a smoke machine to get rid of most of the leaks... (although even before with some the car run great)

Ever since the car never ran right. 

 

73 240z with l28et, z31 300zx maf, ecu, trigger wheel.

Aftermarket FPR set per FSM.  I can visually see fuel pressure raise when revving while standing still. Fuel filter is clean.

 

 Car will drive mostly fine at low RPMs and idles great at 800 RPM.  It lean surges once in a while but every single time no matter what load while driving (from letting RPMS rise slowly to WOT), it will lean and hesitate at 3500 RPM where I let off.   with the car standing at idle I CAN rev past 3500 and the AFR's stay fine. This only happens when im driving. 

 

I tested a known good maf that seemed to fix the problem, only to buy another known good one and the problem persists.

 

Ive replaced the CHTS that tested bad as well.

 

I am throwing a Crank angle sensor code and swapped it with the z31 sensor I got with the rest of the z31 stuff, doesn't change anything.  It gets 12v and the other tests passed.

 

Straight Ohming out the sensor none of the pins have any resistance on either of the CAS's.  Is this normal? Can someone ohm out a known good and let me know if this is the same?

 

I plan on cleaning connectors and such tomorrow and tinkering around but Im pretty stumped.  I feel like its CAS related because of the competely RPM dependent symptom but cannot find any issues with it.

 

Any input would help!

Edited September 24, 2013 by SDgoods

[NewZed]

Raise the fuel pressure a psi or two and see if the problem goes away.  You said it was lean at the problem point.

 

What's your timing doing when you rev?  Have you checked vacuum advance centrifugal timing advances to make sure they do what they're supposed to?

[SDgoods]

Raise the fuel pressure a psi or two and see if the problem goes away.  You said it was lean at the problem point.

 

What's your timing doing when you rev?  Have you checked vacuum advance centrifugal timing advances to make sure they do what they're supposed to?

 

I tried raising fuel pressure a lot more than that and it didnt fix it.  This system doesnt use vacuum advance, it adjusts timing based on the crank angle sensor.    Ill also check what its doing when I rev it tomorrow.

[Milenko2121]

Did you install z31 CAS upside down?

 

Have you tested another working ECU?

 

Where is your MAF placement?

It needs to be at least 12 inches away from the turbo and not near bends to work effectively. The turbo will cause turbulence and the MAF will read wrong.

 

You say you have intercooler piping; are you running draw through(before turbo) or blow through (after turbo) with your MAF?

Edited September 24, 2013 by Milenko2121

[NewZed]

I tried raising fuel pressure a lot more than that and it didnt fix it.  This system doesnt use vacuum advance, it adjusts timing based on the crank angle sensor.    Ill also check what its doing when I rev it tomorrow.

Raising fuel pressure should have fixed a lean problem and possibly created a rich problem, assuming the same injector open times.  The ECCS assumes a certain fuel pressure so it seems like that should have had an effect.

 

It was just a guess.  As for timing, I didn't think through how the Z31 ECCS system works.  It would probably be worth some study though. 

[SDgoods]

Did you install z31 CAS upside down?

 

Have you tested another working ECU?

 

Where is your MAF placement?

It needs to be at least 12 inches away from the turbo and not near bends to work effectively. The turbo will cause turbulence and the MAF will read wrong.

 

You say you have intercooler piping; are you running draw through(before turbo) or blow through (after turbo) with your MAF?

 

There's only one way to put the trigger wheel on I dont know how people manage to install it upside down.

 

I havent tested another ECU I need to find one and do that.

 

The maf is about 9 inches from the compressor but Ive been running this setup for a while now.  it ran rich but physically ran great. Ill try and move the maf farther away from the turbo, however, the other side of the maf also greatly influences turbulence as the air needs to smooth out before hitting the hot wire.  Ive tried a couple variations of piping before the maf but am very limited to room as I have a same sided intercooler. 

 

Running draw through setup, its your typical ET setup.

 

is no one else curious about the fact that its at 3500 rpm every time, no matter of load?  This means that no matter what volume of air (under boost or not), it's leaning out at that exact RPM.  This and the CAS code really makes me think its CAS related.

 

What do you think?

[SDgoods]

Raising fuel pressure should have fixed a lean problem and possibly created a rich problem, assuming the same injector open times.  The ECCS assumes a certain fuel pressure so it seems like that should have had an effect.

 

It was just a guess.  As for timing, I didn't think through how the Z31 ECCS system works.  It would probably be worth some study though. 

 

If everything is working properly thats true.  If its getting a bad signal from lets say, the CAS, it may not be compensating enough, even with the high fuel pressure, to run normal, or even rich. 

[NewZed]

If everything is working properly thats true.  If its getting a bad signal from lets say, the CAS, it may not be compensating enough, even with the high fuel pressure, to run normal, or even rich. 

I'm just a rookie but the only thing that would "use" the RPM data would be the ECU.  The CAS delivers that information, the ECU uses it.  Seems like a map, or look-up table, issue in the ECU program, if it happens exactly at 3500 RPM no matter what (note that the stock tachometers are not super accurate, so 3500 may not be 3500).  The map, programmed for the stock Z31 that the ECU came from, is doing something at 3500 RPM that your L28ET engine doesn't like.  Maybe the ECU is expecting other information at 3500 RPM and it's not there.  The ECU might be switching modes, and getting poor information from the sensors after 3500 RPM.  Or it might be switching modes because it's getting unacceptable information from the sensors. 

 

For example, one mode that switches at 3000 RPM is the injection mode, from group injection to simultaneous.  Another one is EGR at 2700 RPM.  Another one is RPM limiting to 2000 RPM if the MAF signal is bad, which doesn't seem to be your problem.  These numbers are from the 87 FSM.

 

Of course, since you're getting a CAS code, it might just be that the CAS signal is consistently breaking up exactly at at 3500 RPM every time.  They might be the same problem or two different ones. 

 

Just offering a different perspective.  Helps me learn.  Good luck.

[SDgoods]

I'm just a rookie but the only thing that would "use" the RPM data would be the ECU.  The CAS delivers that information, the ECU uses it.  Seems like a map, or look-up table, issue in the ECU program, if it happens exactly at 3500 RPM no matter what (note that the stock tachometers are not super accurate, so 3500 may not be 3500).  The map, programmed for the stock Z31 that the ECU came from, is doing something at 3500 RPM that your L28ET engine doesn't like.  Maybe the ECU is expecting other information at 3500 RPM and it's not there.  The ECU might be switching modes, and getting poor information from the sensors after 3500 RPM.  Or it might be switching modes because it's getting unacceptable information from the sensors. 

 

For example, one mode that switches at 3000 RPM is the injection mode, from group injection to simultaneous.  Another one is EGR at 2700 RPM.  Another one is RPM limiting to 2000 RPM if the MAF signal is bad, which doesn't seem to be your problem.  These numbers are from the 87 FSM.

 

Of course, since you're getting a CAS code, it might just be that the CAS signal is consistently breaking up exactly at at 3500 RPM every time.  They might be the same problem or two different ones. 

 

Just offering a different perspective.  Helps me learn.  Good luck.

 

 

Thanks for the input!  I have a speedhut revolution tach that's pretty accurate.  And whether or not its exactly at 3500 rpm, its at that place on the tach everytime.   

 

Regarding the CAS,  I have 2 seperate sensors, one from the ET and one from the Z31, both of them giving the same exact symptom, which now that you say it, is probably too coincidental.   Ill try and get my hands on another ECU.   I suppose if it got fried and happen to be on the CAS circuit it would throw the same code.  The circuit does recieve 12v and does have a ground but it could still be 'bad'

[SleeperZ]

Is the distributor tight?  Maybe it has a worn out bushing, and you lose the signal at that rpm...

[SDgoods]

Is the distributor tight?  Maybe it has a worn out bushing, and you lose the signal at that rpm...

 

Yes its tight and Ill double check again.

[roye@hrewheels.com]

Daniel what are you using to measure AFR's? If you are seeing a "lean" condition because of extra o2 in the exhaust you could actually have a rich missfire or ignition problem. Also, when you say that you can rev it pass 3500 from an idle you mean in neutral right? Can you hold RPM in that scenario with out droppping AFR?

[SDgoods]

Roy, using my aem wideband. I considered that as well. The plugs do read lean but ill check again tomorrow.

 

Correct I can rev it in neutral and the afr's are fine, but when driving and letting it rev up slowly under small load it leans out just as bad as heavy load.

 

The cap points are getting worn a bit, could that cause this issue? Just seems very coincidental

 

Im testing another ecu tomorrow so that will eliminate that. Still throwing crank senso code and Both my crank angle sensors yield the same symptom. Havent found anything in the harness either.

[billseph]

Just a quick thought regarding your testing. You can switch into different modes on the ECU to clear codes, etc. There is also a mode, mode 5 I believe, don't have my FSM in front of me, where you can see problem codes in real time on the ECU'S red and green LED'S. maybe you could switch into that mode and see when exactly the CAS is acting up. Maybe it's throwing the code at idle (or maybe not) and perhaps you can see it happen when you free rev it to 3500 rpm. It's worth a try.

[SDgoods]

Just a quick thought regarding your testing. You can switch into different modes on the ECU to clear codes, etc. There is also a mode, mode 5 I believe, don't have my FSM in front of me, where you can see problem codes in real time on the ECU'S red and green LED'S. maybe you could switch into that mode and see when exactly the CAS is acting up. Maybe it's throwing the code at idle (or maybe not) and perhaps you can see it happen when you free rev it to 3500 rpm. It's worth a try.

 

The later 300zx's have this, i think 86 or 87 and up. I have an 84 so I dont have the option.

 

I double checked the CAS circuit just now by jumping 2 connector pins and testing at the ECU connector, testing ground circuit, and +12v to the sensor, all of which checked out.   This means that either the ECU or CAS are bad.  My other CAS has the same symptoms, and im testing another ECU here soon.

 

Im basing this off the CAS code that I need to fix either way.  Hopefully these are related and  it fixes this issue and if not, I have some more tinkering to do! 

[Metro]

If it's still screwy after the other ecu, I would definitely check the injectors. I've had dirty injectors cause lean spots at specific rpms (2500rpm, and 4000rpm in my case).

 

Later when I had the original injectors cleaned by witch hunter performance, I was a little surprised to see the before and after only improved by +4-5%. I guess they were screwed up just enough to cause problems.

[SDgoods]

If it's still screwy after the other ecu, I would definitely check the injectors. I've had dirty injectors cause lean spots at specific rpms (2500rpm, and 4000rpm in my case).

 

Later when I had the original injectors cleaned by witch hunter performance, I was a little surprised to see the before and after only improved by +4-5%. I guess they were screwed up just enough to cause problems.

 

Thanks for the input!, I was thinkig about that too.  Regardless ill probably replace/ get them cleaned as half of them are original and half not.   For this issue every thing just seems so coincidental but thats not to say its not possible.

[SDgoods]

Well ECU wasnt it, also picked up another crank angle sensor it seemed a little more lively at low RPMS.

 

I also was too paranoid to slowly rev it past its lean point before, well I got it past 3500 and it stayed lean until around 4k (wasnt wot), then it started to richen up perfectly and went WOT and it pulled and kept good AFRS

 

Is it safe to assume this is an injector problem?  I can get 6 semi new injectors with 1500 miles on them for 100 shipped right now.  Also noticed 3 of my injectors are still brown bodied and 3 are black (replacements?)

 

Still doesnt explain the CAS code which came on immidiately with the new ECU. 

[SDgoods]

Well I have injectors coming.  I figured if its the issue, great, if not, they probably need replacing anyway as 3 were original and 3 were not.  

 

Still very stumped and curious about the CAS code.  3 different sensors, wiring checked out, 2 different ecu's.  What the hell!?

[Metro]

Well I have injectors coming.  I figured if its the issue, great, if not, they probably need replacing anyway as 3 were original and 3 were not.  

 

Still very stumped and curious about the CAS code.  3 different sensors, wiring checked out, 2 different ecu's.  What the hell!?

 

I went through two ECUs, two distributors (turbo and N/A), two different CAS systems, onboard ignitor and GM 4pin HEI module.. and it was dirty injectors. But your CAS error is troubling, it's too bad you haven't fixed that yet.

 

I should have figured my problem out sooner. I had the ability to double check everything via a laptop and didn't pay much mind when my fuel values were increasing by 75-100%. I have a tendency to jump to conclusions when it comes to troubleshooting my car.

 

It stands to reason since you've checked everything out and replaced multiple components that it's somewhere else. You need to start looking at the common elements between them and hopefully it's just the injectors.