I picked up a car that isn't running right. It was originally a 94 v6 that had the harness, ECM, dash, motor and trans put into it from a 1997 ss. Motor has some work done to it and was tuned by pcmforless. Previous owner told me after 3,000 or so miles the drivers side bank was flooding and the injectors were at 136% capacity on that side. I bought it without ever checking any of that over.

I had the car on a lift at my shop two days ago and started messing with it. I don't like running it so idk how it runs when it's warm, while it's cold it pops, backfires and detonates like crazy and will not idle for more than a few seconds. I unplugged the MAF, it ran a bit better so I changed it. Switched o2 sensors around and it still runs like crap (don't see why it would change anything while in open loop anyway). Both sides of the motor flood but the drivers side floods way worse.
I scanned the computer with a snap on modis scanner and see the drivers side o2 sensor isn't reading right for being in open loop (453mv). I bought a new o2 sensor and it still reads off. He had o2 sensor extension wires that he spliced in for the long tubes. I found a short in his splice that had the high or low impedance wire shorting to the ECM or ground wire. I fixed it and nothing changed.

I scanned the car again and noticed some strange things. With the key forward and the motor off, the desired idle is 3,337 rpm and the current idle is 187,456 rpms. I can't access any fuel trim levels either.

I checked over all the grounds and can't find anything else wrong besides that o2 sensor short. I called pcmforless and they quickly and rudely kept talking over me and insisting it was a vacuum leak and would dismiss any faulty ECM readings and blamed my $4,000 scanner for being a piece of garbage. I never dealt with somebody so arrogant and rude... I wish I could have punched him through the phone lol.

I ordered a new ECM from madz28 today and am hoping it will fix my issues! Anybody have any other ideas of what could be wrong with it so I can check it over before my new computer comes in?!

 

You emailed me with this trouble. I'll respond here. Don't pay attention to the desired idle before you start the engine. It is always going to be way high and does not indicate any trouble. You should probably check your O2 wires for the correct power and grounds. Computers don't often fail, but you will know when you get your replacement.

 

I understand and I appreciate the response. I've scanned quiet a few vehicles and never saw anything with a desired idle that high or a current idle above 0 when the vehicle was off. I am not saying you are wrong but it is defiantly not something I have ever seen.

I am a bit confused at this point. I was under the impression that on a cold start the ECM was in open loop mode and didn't use the o2 sensors to compensate for fuel... Mainly relying on the MAF and the computers preset fuel values?

 

Rob is right. Both desired idle and RPM numbers, key on & engine off most always show bogus high numbers. They mean nothing.

 

I understand that but I am baffled by what the oxygen sensors have to do with the issue in open loop. Is one reading low at 453ish mv causing the ECM to think its in closed loop and pull a reading from it?

 

The O2 MV reading wont trigger closed loop.

 

So I can safely remove the o2 sensors from this equation. I can rule out the MAF since I just changed it. I can rule out temp sensors since I validated them reading properly on the scanner.

Two things I actually didn't check are vacuum and fuel pressure but I can't imagine them causing the injectors to geek out the way they are. I have a rich to very rich fuel mixture on every plug; I'd imagine the plug readings would be lean on a few.

I can't really find anybody online who has had similar symptoms without it being an improper air temp reading. One guy says he did similar issues and mentioned changing everything but the ECM and never responded after mentioning the ECM. I am banking on the short on the drivers side o2 wiring burning up the ECM.

 

I'm wondering how you measured the injector duty cycle, and came up with 136%? Do you mean the BLM number for that bank is 136? That is not the injector duty cycle. 128 is neutral, or zero. The O2 stuck at ~450 mili volts would explain that. Personally, I would not have thrown a new MAF sensor at it. Probably only needed cleaned, especially if it had one of those stinking K&N air filters. It is very easy to diagnose a MAF with a scan tool. Didn't need the MAF sensor, nor the O2 sensor.
Re-setting the PCM and unplugging the O2 sensors may fix your richer bank. If not swap sides with the injectors. A good scan tool can do an injector balance test. I use a GM Tech2, it does a great injector balance test.

I tell people to swap sides with O2 sensors instead of simply throwing a new one at it. If the problem doesn't swap sides when you do that, it wasn't the sensor.
Sure beats needlessly running the customer's bill through the roof for nothing.
Not likely it needs a PCM either.

Do you own this shop?

 

OP

you have quite a transplant, including wire harness, with some known (02) wiring issues that you feel you fixed...and possibly some you have not discoverd which may be playing havoc.

I would first confirm signal volts to the 02 wires that were "extended" by PO.

you pushed the button on ordering another ECM from another tuner. At this point put that in and see if it resolves your issues...if not look further into your wiring

fuel pressure, vacum leaks are initial things to check on any engine diagnostic

also the existing Herter tune...was it done for the injectors that are on the motor now??

 

I can not access fuel trim levels. I am going off what the PO had said about it. Even with it running all it says is 0. Any parts I put on this car I can very easily return to Napa other than the ECM. Old MAF was dented into a "D" shape at the opening like it was dropped on the ground and that o2 sensor had previously been soldered back together by someone.

Regardless of what the o2 sensors are reading, what do they do during open loop? What will be different about me unplugging them and forcing the motor to stay into open loop? Already reset the ECM and it made no difference. I never let the car run more than a few seconds. During open loop, even with one o2 reading off does it have anything to do with how the motor will run?

 

You need to properly wire up & mount an OBD2 diagnostic port under the dash so you can scan the data stream from that '97 PCM. Check the service number on your PCM to verify the year model. If it's not the same year model your telling your scan tool, you won't be able to access everything, and some of what you are seeing may be bogus. Be certain you have the Class 2 data terminal is connected.

NAPA, and any other parts store, won't want to take back parts that have been installed and run. They need to be able to sell them again. Who would want to go to a parts store and be sold a used part? Pretty much nobody.

 

Ed,
This car has a proper obd 2 port. Their was no splicing of the harness (other than the o2s and the new MAF splice). PO simply took the entire harness from one car and put it into another... He even changed the dashboard over. He did very descent work for the most part, he is an ASE certified technician at a Nissan dealer.

I validated the module service number (16242921) as a 1997 ECM. I can communicate with it to check almost everything but the fuel trim. I actually tried to use the scanner to test several things such as the MAF, clutch on the AC compressor and various other things and each time the scanner locked up on me and returned me to the main screen.

And believe it or not, the local Napa is very lenient with you when you do thousands of dollars a month in parts. I appreciate your help. This car really only gets looked over in our spare time and it's kind of a pain pushing it in and out of a bay. And to answer your previous question it is one of my personal vehicles.

 

I'm also an ASE tech, with the L1 cert (Advanced Diagnostics & Drivability ), and when I closed my shop I was doing thousands a month with NAPA. I would never ask them to take anything back like that. I do my diagnostics to verify the parts are needed before I order them. It just isn't the right thing to do. I have been in there when DIY guys would be trying to do that. I know what they think about it.
If you are buying enough you might ask about their Master Installer program I was on. Deeper discounts. I don't remember the required dollar per month threshold. Be worth asking about if you really spend enough with them. I buy about everything from NAPA.

If you can't access all the data, he may not have all the terminals in the diagnostic connector wired correctly, or need a better scan tool. NAPA used to sell the Tech2. Nothing else like it for GM vehicles. Got both of mine through one of my contacts at GM. $38,00.00 ten + years ago. No telling now.

 

You say it "Flooding" out the cylinders, if this is a true statement, have you checked your FP Regulator?

 

I looked over the car some more today.

Vacuum is good. Fuel pressure stays at about 41ish cranking and doesn't bleed down afterwards. While running the pressure sits at 37 psi. I did not pull the rail and see if any injectors are bleeding but fouling across all 8 in open loop would make me assume their isn't any possible way all 8 are stuck open.

By flooding I mean the plugs are coming out extremely fouled, soaked in fuel and within seconds of running the oil stinks like gas.

Now that I am on a computer it is a little easier to type. The car fires right up, holds a somewhat descent idle for a few seconds (although you can tell it isn't running right) while it detonates and occasionally backfires. At this point I usually chicken out and kill it before the detonation gets any worse.

As previously stated, I found a short on one of the o2 sensor wire extensions (since the car has long tubes) and the purple wire was shorting out on the ground wire. PO mentioned the car started running rough after a few minutes of running and drove it about a mile home. In the following days while diagnosing it he said the issue got worse. Wouldn't that short be enough to mess up the computer?

 

No that likely didn't hurt the computer. I would be concerned about it detonating while just sitting there idling. In all my years, I don't believe I ever witnessed that. If the spark advance is OK on the scan tool, I would rig up a timing pointer to verify the indicated spark advance is correct. If the cam is in one out of time one tooth advanced, you can add 22 degrees to what your scan tool shows. If that is the case, it's actually idling around 38 to 48 degrees.
If fuel pressure holds half hour or so after shutting it off the regulator diaphragm can't be leaking fuel into that vacuum hose to the manifold.

I'm guessing you verified the coolant and IAT temps while the car was dead cold with your scan tool?

MAP numbers are correct? Little arrow on the MAF sensor is pointing toward the TB, not the air filter? Backwards MAF = rich mixture. Not as bad as you did rive here, but could contribute.

Have you verified the injectors ?

 

It randomly detonates very badly. It almost sounds like it spun a bearing randomly at times. Other times it makes no noise at all. It terrifies me, it has forged pistons but I doubt they will be any good with the abuse it's getting. Plugs aren't showing any signs of damage yet.

I will have to check timing, what would allow it to jump (if the PO had installed it correctly)? I was thinking about it, can the rotor backing loose cause it to run like this?! I can't see it being at 38-48 degrees honestly, it doesn't have a brutally hard crank to it when i start it.

I also didn't see any fuel in the vacuum line off the regulator.

Checked the temp sensors on the scanner and they aren't reading any thing astronomical. Haven't checked the map numbers. MAF is facing the right way.

I think at this point I am going to wait for the new computer before I mess with it anymore. Next few days at the shop are swamped so unless it is remotely warm out I refuse to work on it outside. Thanks for the help, I will get back to you in a few days!

 

Have you verified the injectors are what it is tuned for?

A good way to verify the temp sensors is after sitting overnight, or all day, so it is "cold soaked". Hook up your scanner, turn the key on, don't start it, just verify that both temp sensors show ambient. 65 degrees where you are, they should both show 65 degrees, or very close. Ten or more degrees one way or the other is a problem. Then warm it up checking the IAT sensor area and thermostat area with a temp gun to compare to scanner indicated values.
MAP KPA, key on & engine off should be at least 99 at sea level, 100 or so is common. Idling is typically 35 to 65 KPA depending on camshaft & compression.

Have you checked actual manifold vacuum? Subtract manifold vacuum from the barometer value, that is what the MAP should be at idle. That verifies the MAP sensor is correct. Small airports are good sources of barometer. Pilots use that to set their altimeters before take off. Check your MAP reading, engine off, to the airport Baro. Subtract your manifold the Baro number from the airport. Your idle MAP on your scan tool should be very close. If your scanner number is much higher, it will cause the car to run rich.

Do you know for SURE what the injectors are?

 

Little update with a quick recap...
Found shorted O2 sensor, 1 rusted O2 sensor, bad MAF, pulling air after MAF, poor grounds (weren't bare metal).
Anyway, ECM did change things a little bit. Car ran significantly smoother without the detonation sound but overall, as Ed had stated, wasn't the issue (completely at least). Noticed the MAP sensor was causing a vacuum leak. I chased just about the entire engine harness for obvious signs of damage one wire at a time.

So my buddy was tinkering around with it today and fired it up. With the misfire very obvious he managed to keep it running (without detonating) long enough to get the SES light to flash at him! The car died out of nowhere and wouldn't start.

I figured it was time to change the oil and while it was draining I decided to look over the firing order... the story I was told was the PO had it running and it started running like crap (I assume the shorted O2 wiring); he NEVER mentioned doing plugs and wires. Anyway, I looked over the firing order and noticed plugs 5 & 7 were backwards (only took me 16 hours to notice it lmao).

Feeling on top of the world that I had checked over everything and fixed any issues, I get in the car and turn the key... nothing. I check for spark... good. I hook a fuel pressure gauge up and turn the key... I hear the fuel pump kick on but it wasn't reading on the gauge. I take the fuel line off before the filter and turn the key... nothing. I take the pump out of the tank and bench test it in a bucket of water... it turns on but doesn't suck.


Now I'm certain I have fixed everything, very upset about the fuel pump dying today but on Wednesday all should be good

Big thanks to Ed Wright for all of the help, although stubborn 23 year old me didn't want to listen... could have probably saved $185 on the retune but at least now I know it's tuned for everything on the car (new computer did stop the detonating and fouling of the bulk of the plugs).

 

Please update if the fuel pump fixes everything, ive been having the same exact problems(almost word for word) with my project car. Same fuel pressure and idle misfire.

Glad your getting the bugs worked out!