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Engine running very very poorly - ideas?

This is a discussion on Engine running very very poorly - ideas? within the General Help forums, part of the LSx Technical Help Section category; Hi guys, I was hoping there is a mechanical genius out there who might be able to give me some ...

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    Engine running very very poorly - ideas?

    Hi guys,

    I was hoping there is a mechanical genius out there who might be able to give me some clues why my car s running so badly.

    I'm running an LS1 corvette '99 engine in a AC cobra recreation.

    The engine generally starts and runs fine for 20 minutes then it starts misfiring and increasingly looses power until it finally stops after another 10-20minutes. If I then leave it for 30 minutes it runs fine for a while before all starting again.

    I'm showing plenty of codes but due to the bespoke nature of the car I don't think most are relevant.

    P0300 (when it is misfiring) - Random/Multiple Cylinder Misfire
    P0101 - MAF Circuit low input
    P0405 - EGR Flow sensor A circuit low input
    P0412 - Secondary Air Infection System Valve A malfunction
    P0418 - Secondary Air Infection System Relay A malfunction
    P0443 - EVAP Emission Control system Purge valve C fault
    P0449 EVAP Emission Control system vent vlv/sol mlf
    P0452 - EVAP Emission Control systm Pres Sensor Low
    also getting some tranmission ones which I don't think are relevant


    The EGR has all been removed/bypassed, the rear O2s removed and no catalytic coverter - so I think that explains most of the codes. I cleaned the MAF with electric contact cleaner although code still there.

    Sadly I have no means to get live engine data, just read codes.

    My thoughts so far -
    Broken MAF / sensor - surely this would be broken all the time though
    Dodgy / over heating coils? (the spark plugs look prettry clean and are fairly new - (NGK PTR6F-13 I hope a good plug) so not quite sure why the coils would pack in)
    Dodgy O2 sensors (front as I don't have rears)
    Failing fuel pump or fuel regulator.

    Short of replacing all of these parts in the hope it solves the problem I'm not quite sure what to do.

    Any thoughts?

    Thanks in advance
    Adam
    Last edited by cullywoo; 08-12-2012 at 09:34 AM.

  2. #2
    Smiles for 9.5 Years cammed goat's Avatar
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    As far as the MAF goes, has it ever been cleaned?

    Almost sounds like a wiring issue. Are all of the connectors tight? Are all of the grounds intact and connected properly?

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    I cleaned the MAF with electrical contact cleaner (can't get the MAF cleaner here)
    The wiring seems ok but some of it is a bit amateur so would entirely suprise me. Connectors at coils seem ok. A lot of wires to trace - although I might start working my way through it.

  4. #4
    Smiles for 9.5 Years cammed goat's Avatar
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    Ok. There could be a ground that isn't connected or not connected properly.

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    I agree, i think you should pull all the connectors that you are suspect of, and clean, then reinstall.

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    Moderator Firebirdjones's Avatar
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    Is this a homemade harness for the retro swap? If so then I agree with these guys, there may be something missed (very easy to do) and it will cause gremlins like that.

    I've done retro swaps like this before and after a couple headaches like this I decided to use custom made harnesses for the application, never another electric gremlin.

    Look over everything close. Grounds as mentioned are the easy things to look for. After that you'll need to look inside every single weather pack connector, sometimes it's a single wire that wasn't seated completely and pulled out that isn't making contact, including the main plug at the computer. From there you might have to start tracing wires and check for a pinched wire or one that has a screw going through it.

    It's a pain and it will definately take some time.

    Edit: neat swap by the way that has always been in the back of my mind. I used to ask Superformance (when I lived out there) in the late 90's when they would make a chassis to adapt the new LS engine and always got funny looks.
    10 years later I ran the idea past Factory Five and they told me they had thought about making the mounts for these swaps, so they were at least open to the idea. Figured some day I'd buy the Factory Five kit car when my wife totals her current 4th gen and just yank the drivetrain out of it
    Last edited by Firebirdjones; 08-12-2012 at 02:22 PM.

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    expensive tires az gt eater's Avatar
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    Quote Originally Posted by Firebirdjones View Post
    Figured some day I'd buy the Factory Five kit car when my wife totals her current 4th gen and just yank the drivetrain out of it
    Trying to say shes a scary person to ride with? Sorry Firebird, just got a kick out of that one.

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    Moderator Firebirdjones's Avatar
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    Quote Originally Posted by az gt eater View Post
    Trying to say shes a scary person to ride with? Sorry Firebird, just got a kick out of that one.
    Nope,,,,,it's all the tailgaters in AZ. Everyone drives like their ass is on fire. Last 4th gen was totaled by a tailgater, shoved the trunk into the back seat. I figure it's only a matter of time before it happens again Part of the reason why alot of my stuff doesn't come out,,,don't get me started on that rant

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    Quote Originally Posted by Firebirdjones View Post
    Everyone drives like their ass is on fire.
    Now you are talking about me. Except I don't tailgate. I am always out front. I even use my blinker without exception. Now, don't get ME started on that rant.

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    Veteran 0rion's Avatar
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    I would first start with those relevant codes and try to find out what's behind those. Is your air injection system hooked up? If it's not then you can throw those codes away. The main culprit I see is that MAF code.....you'll want to get to the bottom of that one but even if it does fail it should still go to SD and run. Has any work been done on the engine at all? Is 20 minutes an accurate time for when it starts? Reason I ask is the car will go to closed loop when the coolant temp reaches ~100 degrees. I'm wondering if that's when your problem starts. What's your coolant temps look like?
    This stuff is much easier to troubleshoot if you have something like hptuners to look at live data and get a picture of exactly what's going on with the car.

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    Not to jack the thread but whats the proper way to clean yor MAF? Iread a post on here somewhere to use CRC MAF cleaner and something about removing the sensor first?...Is the sensor in the MAF?..or the one in the lid?

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    the MAF is between the lid and the throttle body. It'll have 3 very thin and fragile wires with resistors in them that run across the MAF. You want to spray those down with MAF cleaner. I would be careful using other cleaners because they may leave a film on the wires. Those wires heat up and as the air flows over them they cool down....this temp drop tells the car how much air is flowing through the MAF.

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    Thanks guys. This uses the standard wiring harness that came with the engine, although many bits have been blanked off.

    The air injection system has been removed.

    If the MAF was causing the problem, if I unplugged it would it resolve the problem (although run rich)?

    No recent work other than a change of plugs - although that was a couple of months ago.

    Orion, The temperature is generally stable - I've noticed the water temp is 90 degrees as a rule when the problem starts. The timing after starting is variable - usually about 20 minutes. Sorry you'll have to explain about the 'car going to a closed loop'.

    I admit live data would be great, not many people over here know much about these engines. I guess I could get some software and try myself, but I suspect the data will be a bit baffling.

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    when the coolant temp hits, I want to say 90ish I would have to look it up but low 90's anyhow, the car goes into "closed loop". That's when it starts using feedback from the O2's to control fueling. What would be nice to see is your coolant temp and fuel trims to see exactly what the O2's are seeing fuel wise when it goes to closed loop. Keep in mind you're dealing with a transplant so the door is completely open on where to start looking.....it's not quite the same as trouble shooting an fbody that has the engine that came with it. Like others have stated.....it could be ground or harness related but that 101 code is something you'll want to get to the bottom of. If 101 is thrown the car is in SD anyhow and not using the MAF so unplugging it *shouldn't* have any effect. The reason I initially asked about the coolant temp is because coolant temps will have a direct effect on fueling calc's. An example would be my buddies vette a few years ago....it would start and idle fine. After a few minutes it would load up and basically flood itself out. Ended up being a bad coolant temp sensor....it was showing some crazy cold reading and when the car went to closed loop it would dump fuel. Like I say...it would be nice to watch that coolant temp right when it happens and make sure the reading makes sense along with long term fuel trims to see what percentage they're showing.

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    Orion, I may get a scanner for some live data about what's happening temp/fuel wise but the picture your painting sounds like a plausible explanation.

    I admit the donor element (and not one I installed) really makes diagnosis difficult, cold be any number of installation floors. One major one I've picked up is the absence of a right side O2 sensor, I've looked and looked and there doesn't seem to be one. The left one is there - I wonder if this has been set up to use one sensor for both banks. Can this work?

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    You can put the car in open loop and not run the O2's at all but the symptoms you're describing sounds like it's going to closed loop. In that case you have to have both of those front sensors. I think you may be getting warm on diagnosing your problem.

    Is the car using stock manifolds? That sensor will be pretty high up on that manifold if memory serves me correctly.....been years since I've even seen a stock manifold.
    Last edited by 0rion; 08-14-2012 at 11:33 PM.

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    Moderator Firebirdjones's Avatar
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    Hmm,,,,would that depend on the year and style of manifold used Orion?

    Since it's a cobra I would assume he is running the sidepipes (seems everyone does) so it begs the question..........Are you running a retro swap header configuration for this?

    When I've done my LS swaps I've always had to pick a spot on the system as close to the manifolds as possible and weld my own bungs in there. The truck manifolds I used on my most recent swap didn't have provisions for 02's. An aftermarket set of headers would have though....

    Are there any LS OEM manifolds out there that have 02 provisions?
    Last edited by Firebirdjones; 08-14-2012 at 04:42 PM.

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    I know on my car, if the intake tube comes off the tb, my car does a few second after.


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    Quote Originally Posted by Firebirdjones View Post
    Hmm,,,,would that depend on the year and style of manifold used Orion?

    Since it's a cobra I would assume he is running the sidepipes (seems everyone does) so it begs the question..........Are you running a retro swap header configuration for this?

    When I've done my LS swaps I've always had to pick a spot on the system as close to the manifolds as possible and weld my own bungs in there. The truck manifolds I used on my most recent swap didn't have provisions for 02's. An aftermarket set of headers would have though....

    Are there any LS OEM manifolds out there that have 02 provisions?
    I think for the most part those LS manifolds are pretty similar between the vette/f-body as far as where the O2's are located. I'm pretty sure they're just before that collector which puts them up higher. It's been so long since I've seen either though I wouldn't feel comfortable definitely saying that's correct. Either way though if he's running the vette pcm and it's not set up open loop he'll need both those sensors functioning.
    Last edited by 0rion; 08-14-2012 at 11:34 PM.

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    Moderator Firebirdjones's Avatar
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    Quote Originally Posted by 0rion View Post
    I think for the most part those LS manifolds are pretty similar between the vette/f-body as far as where the O2's are located. I'm pretty sure they're just before that collector which puts them up higher. It's been so long since I've seen either though I wouldn't feel comfortable definitely saying that's correct. Either way though if he's running the vette pcm and it's not set up closed loop he'll need both those sensors functioning.
    Yep I agree. Actually I was a little shocked when he said he see's only one sensor installed. When you led him to the closed loop running issue with temp, I figured that was on the right track. I expected to hear about a bad 02 or a loose wire,,,,,not an 02 missing completely

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