Megasquirt (?)

[dhen]

Hi all,

I've exhausted my patience. My car runs fine, until I get over 4,000 RPM on the road. Then it stumbles like it's running of gas. Revving it at idle is not a problem.

This is what I have and what I've done, none of which have solved the problem:

1) temp sensor with 1,000 miles
2) Walbro 255 fuel pump with 2,500 miles
3) VPC (MAF delete) - I set it a little richer and this helped a little, but didn't solve the problem
4) set the timing to 15 degrees with a timing light
5) swapped out the ECU with a code 76 ECU (UKDM) from the code 98 ECU I was using
6) bumped up the fuel pressure to 40 PSI
7) replaced the coil packs, ignition module, and dropping resistors with known good spares
8) O2 sensor with 2,000 miles
9) swapped out the injectors with known good spares
10) swapped out the TPS
11) swapped out the CAS
12) New air temp sensor for VPC
13) New MAP sensor for VPC
14) New Nismo fuel pressure regulator

Here's the strange part: My wide-band O2 sensor says I'm not leaning out when it stumbles. I'm beginning to think that my VPC is the problem.

Do I need a new engine management system? If so, which one would you recommend? I'm leaning towards Megasquirt, but honestly, I just want my car to work.

I have a limited budget, since this isn't my daily driver.

Thanks,

Darian

[themadscientist]

I had success with a VPC, but honestly, the thing is like 15 years old. Time to quit playing games and ditch that prehistoric s***. All my vintage gear is in a box now waiting to be put in a display case. I can't justify trusting it to run my engine anymore.

[dhen]

Before I replace it, is there anything mechanical that could be causing this problem? I think I've narrowed it down, but I'm not sure.

Darian

[blownhemi]

I had success with a VPC, but honestly, the thing is like 15 years old. Time to quit playing games and ditch that prehistoric s***. All my vintage gear is in a box now waiting to be put in a display case. I can't justify trusting it to run my engine anymore.

What's a VPC? Why have your replaced your MAF with it?

Before I replace it, is there anything mechanical that could be causing this problem? I think I've narrowed it down, but I'm not sure.

Darian

I had the same problem recently, I was overboosting because of a leaky wastegate. Engine cut out 4500-ish in 2nd. Under load and boost, that is. Did you try reaching the 4000 stumble point under both high and low load? Any difference?

[dash]

i believe VPC is the HKS vein pressure converter.... basicly a conversion to map sensor
aren't they "chipped" for specific injector size..... u got correct ones

what's the boost doing during all this ? no leaks ?

[themadscientist]

They use a pressure and air temp sensor to deliver an air flow signal to the stock computer. There were several off the shelf chips for common combination that would allow you to get close enough to use the adjustment knobs to fine tune it. The chips could be reflashed like an ECU's ROM.

If you look at the rear of the intake you will see a sensor on the firewall supplied by the red hose. That's the pressure sensor. In the side of the surge tank at the very back you will see a sensor screwed into it. That's the air temp sensor.



Compared to running with the AFM, the response and power was noticeable. Of course that might be from removing the S14 turbo and putting on the T04E,,,naaa.

The device on the right bottom is the VPC. Above it is the Graphic Control Computer, GCC, that adds RPM-range fine tuning. The basic VPC only has a gain knob that cranks the whole map up. It says PFC, but that's because the same GCC could be plugged into an F-CON.

[dhen]

i believe VPC is the HKS vein pressure converter.... basicly a conversion to map sensor
aren't they "chipped" for specific injector size.

That's correct.

Well, I might be closer to an answer. I just got back from driving and my boost level goes down a couple of PSI when it stumbles. My wastegate is pretty beat up, is this the possible answer? How do you check for a boost leak? Do you use smoke?

My car overheated a few months ago when I had a coolant leak that I didn't catch in time. I got the head resurfaced and put on a new head gasket. Is it possible that I damaged the turbo when this happened? It's aT28 so it's got oil and coolant.

The most important reading - my oil pressure - is good. The stumbling isn't completely consistent. It's often OK at 6,000 in 1st gear, but 2nd gear is worse. A coincidence?

Any replies would be much appreciated.

[themadscientist]

A stumble would cause the exhaust flow to become irregular and slow down the turbo. Which is the cause and which is the symptom may be difficult to discern.

[dhen]

A stumble would cause the exhaust flow to become irregular and slow down the turbo. Which is the cause and which is the symptom may be difficult to discern.

Thanks. I know you can't diagnose this over the internet, but if you were in my shoes, what would you look at next?

[boost_boy]

Check for boost leaks. No boost leak, I'd ditch the magical box (VPC). I mean seriously, those things were pain the butts when they were popular back in the day. Put the car back to stock and go from there. I would seriously avoid the going megasquirt unless you have someone close by that can set it up for you "correctly".

[themadscientist]

I'll do my best.

Revving in neutral is not the same as the same RPMs under a load. It would seem that the problem is a "weakness" rather than an outright failure.

I always go back to the basics when trying to diagnose a problem. The combustion triangle, fuel, air and spark.

You seem to have adequate fuel with all seemingly good components and you mentioned the static pressure so I assume you have a gauge, but what is the pressure when you are driving? I had a Skyline that would hum along happily until you got on it and then the car would slam into what I described as an "invisible wall of jello." It would fall flat on its face under even a moderate load. It took monitoring the operating fuel pressure to catch it. The static pressure was strong and it revved great, but when the boost came on rolling down the road the pump could not meet the increasing demand and rail pressure dropped to near carburetor-friendly PSI.

You have changed the ECU and all the major ignition components so I think it safe to assume your spark is adequate. If your plugs are coming out dry you are good for spark.

Now air. The VPC could be bad as it has been a constant throughout your many parts interchanges. Do you have an AFM you can plug back in to check?

You mention 4000 RPM as the tipping point where it acts screwy. Is that always? Is that 4000 regardless of whether you slowly accelerate or nail it?

Does the boost level affect where it starts to get goofy? If you turn the boost down can you run a little higher?

Have considered that you might have lightly blown the head gasket between two cylinders and with enough load the pressure bleeds between them?

Try unhooking the turbo's outlet pipe to the intercooler and doing a test drive in NA mode. I am willing to bet ryans a**-cherry the problem either goes away or mitigates a noticeable amount.

[dhen]

Thanks! That will keep me busy for a while! I'll let you know when I've checked it all. Here's what I can tell you so far.

The head gasket is new and the head was resurfaced. I will do a compression check and see if that shows up anything.

I tried the stock MAF a while back. It didn't help.

Lowering the boost seemed to help a little.

I have to test the other stuff, though. I'll get back to you.

Darian

[themadscientist]

Static pressure may be ok if the gasket is just leaking under boost. Unhooking the turbo pipe and running NA might show that if the problem goes away.

When my CA failed I lost pressure on #1. It would not start because of it. If I squirted oil into the cylinder to coat the walls it imparted enough sealing improvement that the engine would start and run sort of ok until it warmed up and the parts expanded then it ran fine, no stumbles it was just down on power. It doesn't take much to make things act wrong.

[dash]

what plug
what gap ?

[blownhemi]

Check for boost leaks. No boost leak, I'd ditch the magical box (VPC). I mean seriously, those things were pain the butts when they were popular back in the day. Put the car back to stock and go from there. I would seriously avoid the going megasquirt unless you have someone close by that can set it up for you "correctly".

Wouldn't a boost leak cause him to run rich with MAF, and have no effect whatsoever with his current, MAP-based system (besides no oomph)?
He would also run rich if his spark plugs were in any way faulty, would he not?

Can you tell if it starts to lean out before the stumble comes on, or the other way around?

Take the hood off, and check the fuel pressure while the problem appears. If you can't see the gauge from your seat, rig up a camcorder/webcam&laptop/mirror/friend-leaning-out or something to that effect. (I'm assuming, your gauge is under the hood, since you've only mentioned static fuel pressure.)
But with all things new, pump, FPR, this probably shouldn't be a problem. You're running 10psi, and the DSM 450cc injectors, right? Did you get it from a reliable source, are they definitely 450cc injectors? Because ~10-ish psi with a more efficient turbo is just about the limit for the stock 370cc injectors. Mine maxed out at 11 psi on an ebay T3/T4.

[dhen]

I took the car on the road and tested it one more time. The sputtering problem is intermittent, but is a problem 80% of the time. My air/fuel ratio is in the 10s when boosting.

Checked the compression and got 152-150-151-151

I forgot to mention that the fuel pump is about 5 years old, but with little mileage. The fuel pressure looked OK on the road, though.

Try unhooking the turbo's outlet pipe to the intercooler and doing a test drive in NA mode. I am willing to bet ryans a**-cherry the problem either goes away or mitigates a noticeable amount.

It looks like you won the bet. (Does Ryan know about this?) I drove with the turbo disconnected from the intercooler, and it didn't sputter at all. I'm not 100% sure what this tells me. It sounds like it means my problem is mechanical and not electronic. Am I right?

You mentioned the idea that the head gasket might only be leaking under boost. I installed ARP head studs when I replaced the head gasket. The head rubbed against the socket pretty bad - it was a pretty tight fit. I torqued it to 80 PSI as per the instructions. Is it possible that the resistance of the rubbing against my socket caused me to believe I was torquing to 80 when in fact it was lower?

The only other interesting thing is that I saw a small puff of smoke for a second on the driver's side. (The hood is off until I sort this out.) I thought - Ah ha! Then looked closer and there is a small oil leak where the oil goes into the top of of the turbo, so I think that was a red herring.



The spark plugs are dry and look good. Although the one on the right, from cylinder 1, is slightly white.





So great, I've got my car running but I can't boost it. What fun is that? Well, at least I got to teach my son how to do a compression check...

Any advice would be welcome.

Darian

[float_6969]

I was NOT aware of the bet. But don't let mad fool you. We traded azz-cherry's years ago. Sorry I'm in on this so late, but to me it sounds like boost cut from the stock ECU. I would be willing to bet TMS's third testicle that if you were to measure the MAFS (VPC output) voltage when the cut occurs, you're well over 4vdc. If this is the case, the stock ECU think's that you're overboosting and is cutting fuel. This is why piggybacks can be a pain.

I don't think it's hardware, I think it's software. Although you basically tested this by running N/A, you can also test this by lowering the MAFS voltage. There are different ways of doing this. The first and easiest would be to lower the boost you're running. If you're only running wastegate pressure and it's an internal wastegate, simply unhook the wastegate arm from the actuator. This will run little to no boost until you get pretty high in the revs and a lot of throttle. If this solves the problem, then I think you need to re-tune the VPC or re-flash the ECU or both. The second way you can test this is to lean out the fuel. Running 10:1 AFR's is unnessicary and just wastes fuel, drives the MAFS voltage up, and all kinds of other not-fun- things. Lean it out to AT LEAST 11.5:1. You can go leaner, but you have to start to watch for detonation at higher revs/boost levels if your intercooler isn't keeping the intake temps down. The last option, is the one I mentioned before, which is to actually measure the voltage coming out of the VPC. This is the most accurate and will definitively confirm/deny my suspicions, but it's also the most work.

[themadscientist]

You were always jealous of that third nut, hater.

As an aside, WTF is with that #1 plug? That is a significant difference from the other ones. You need to figure out what that's about too.

[boost_boy]

Check for boost leaks. No boost leak, I'd ditch the magical box (VPC). I mean seriously, those things were pain the butts when they were popular back in the day. Put the car back to stock and go from there. I would seriously avoid the going megasquirt unless you have someone close by that can set it up for you "correctly".

Wouldn't a boost leak cause him to run rich with MAF, and have no effect whatsoever with his current, MAP-based system (besides no oomph)?
He would also run rich if his spark plugs were in any way faulty, would he not?

Can you tell if it starts to lean out before the stumble comes on, or the other way around?

Take the hood off, and check the fuel pressure while the problem appears. If you can't see the gauge from your seat, rig up a camcorder/webcam&laptop/mirror/friend-leaning-out or something to that effect. (I'm assuming, your gauge is under the hood, since you've only mentioned static fuel pressure.)
But with all things new, pump, FPR, this probably shouldn't be a problem. You're running 10psi, and the DSM 450cc injectors, right? Did you get it from a reliable source, are they definitely 450cc injectors? Because ~10-ish psi with a more efficient turbo is just about the limit for the stock 370cc injectors. Mine maxed out at 11 psi on an ebay T3/T4.

If he's losing pressure at the intake manifold or the seals between the intake manifold's butterfly plenum, it will affect his performance indeed; especially on his map-based VPC set-up. A boost leak at the intake manifold would cause you to run rich on a map-based system as well and I know this because I've experienced it. When you get tired of fighting with gremlins, remember, ditch the vpc and see how she goes. And that's my final answer

[dhen]

I was NOT aware of the bet. But don't let mad fool you. We traded azz-cherry's years ago.

Oh well, no worries there. I was thinking this was some really high stakes bet...

Well, this sucks. Now I have to get a new engine management system. I knew this day would come with the ancient stuff I'm using, but I was hoping it wouldn't.

Hopefully, I get get some money for the stuff I have to offset the cost.

Not looking forward to getting it tuned, either, since I got burned doing that once. Like I said, though, this day was bound to come.

Thanks for all the input, though. I really appreciate it.

Darian

EDIT: Is there any way the butterflies could cause this?

[float_6969]

The seals, if they're leaking, might cause some problems, but I doubt it would cause the issues you've having.

[dhen]

That wasn't what I wanted to hear, but thanks for the honesty. Seriously, I appreciate it.

Darian

[dhen]

If you're only running wastegate pressure and it's an internal wastegate, simply unhook the wastegate arm from the actuator. This will run little to no boost until you get pretty high in the revs and a lot of throttle. If this solves the problem, then I think you need to re-tune the VPC or re-flash the ECU or both.

This solved the problem, so you were right. Before I dump a sh!tload of money into a new engine management system, I need to ask. Do you think there is any way that this could be the boost actuator itself?

Thanks,

Darian

[themadscientist]

The unhooking of the actuator was just to reduce the boost for the test. If your actuator is acting weird it would not cause the running problems you initially described. Boost would just be erratic. It sounds like your engine is not addressing the boost as it's supposed to, not that the boost is absent or erratic.

[dhen]

Thanks for letting me know. Looks like I've got some downtime, but I WILL get this fixed.

This car's been in the family since 1964, so I can't give up that easily...

Take care,

Darian

[float_6969]

OK, so if the issue is that we're hitting the cut on the MAFS input/vpc output, then this shouldn't be too big of an issue to deal with. If I'm thinking about this correctly (it's late, so I could be wrong), if you chipped the ECU for a bigger MAFS or installed bigger injectors, then turned the VPC down, this should solve your issue.

This is my logic. (the following numbers are made up, so do take them to heart). If in your current situation, you hit boost cut at 15psi, and the VPC is outputting 5vdc with the knobs the way you have them set, then what needs to happen is that we need to modify the system to lower the VPC output at 15psi. To do this we have to turn the knobs down. This will cause the voltage to drop. This will have 2 effects. The first is that boost cut will go away, which is what we want. But the other is that it will reduce fuel, which we may not want. (I don't know what your afr's are like). So to ofset the decrease in fuel from lowing the voltage, we have 2 options. The first is to install bigger injectors. The second is to chip the ECU for a bigger MAFS. This will cause the ECU to expect a lower voltage at the same mass air flow, solving the boost cut issue.

Remember that you're dealing with a piggy back here, so we can't get too far away from stock before we really get into the side effects of altering the MAFS voltage on timing.

Have you had it on a dyno yet? How much more power do you want to make? I know the info is on the boards somewhere, but what all has been changed from stock? What are your AFR's like? Where do you have the knobs on the VPC set?

[float_6969]

OK, I just re-read the intire thread and saw that you said your AFR's are going to 10:1 when you get into boost. That's way to high, and driving your VPC output up. Turn the enrichment down so that you're in the 12:1 range. As long as you're on good 91 octane pump gas and have a good intercooler setup, there is no need to run that rich.

[dhen]

Thanks for your response, Ryan.

The thing is that this setup was working with the same settings before. If I understand you correctly, to fix it I have to trick the computer that tricks the ECU. Even if it works, there's an error in there somewhere and unlike a fine wine, this system isn't getting better with age.

Unless I find out that there is something mechanical going on, I don't think it makes sense to keep this engine management duct-taped together.

[dhen]

I found out that my exhaust manifold is cracked on the 4th runner that goes to the turbo. It was in a place that was hard to see. Could this be my problem? Here's a picture:



And another one.



As I said the problem is intermittent. I had all the parts in a box when I got the engine rebuilt. The alternator was banged up, I guess the manifold was too.

[float_6969]

No, that shouldn't cause the problem you're having. If it's possible, put a voltage meter on the output of the VPC. If you can tell us what the voltage is when the cut is happening, I can tell you for sure if the problem is just the STOCK ECU doing it's job and cutting fuel, or if there is something else going on.

If the voltage is getting high (over 4vdc IIRC), then I can't answer why it worked before and it doesn't now. What I can tell you is that if you re-chip the ECU (not the VPC) for a bigger MAFS, then turn the VPC knobs down, you won't have the cut anymore.