q-jet problems - Team Camaro Tech
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post #1 of 15 (permalink) Old Nov 3rd, 06, 11:49 AM Thread Starter
 
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q-jet problems

just bought an '84 z28 with what i think is a carb problem. the engine will start right up and idle fine. however, if i rev the engine any higher than about 2500rpm and hold it stead it will be okay for about 20 seconds then will start sputtering, jerking, and backfiring, and drop rpms even though the accelerator hasn't moved. if i rev it up to 3500 for 20 seconds it will drop to somewhere between 2000 and 2500rpm. i can let off the accelerator and it will return to idling just fine. then i can bring it back up to 3500 and after another 20 seconds it does it again. I'm no mechanic so any suggestions would be greatly appreciated. thanks
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post #2 of 15 (permalink) Old Nov 3rd, 06, 12:09 PM
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Re: q-jet problems

Could likely be fuel delivery to the carb.
Check;
Fuel filter
Hoses for deterioration
Fuel lines for damage
Pick-up and filter sock (in tank) for problems like plugged filter sock

You could splice a fuel pressure gauge into the delivery hose after the fuel pump and monitor the pressure as the throttle is opened.

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post #3 of 15 (permalink) Old Nov 3rd, 06, 01:39 PM Thread Starter
 
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update and additional information

thanks vintage...replaced fuel filter, checked all fuel lines, checked hoses, all pretty good shape. i'd like to throw a few other pieces of information out there...the car will run fine as long as the rpms are low. i can drive it around the neighborhood 25-35mph all day long. when it gets up to highway speed is where the problem occurs. thanks
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post #4 of 15 (permalink) Old Nov 3rd, 06, 03:23 PM
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Re: update and additional information

If you open your gas cap when it starts running bad is there a vacuum in the tank? I just thought of it because I have an 87 that runs crappy if you put the cap on tight. It should be vented through a charcoal canister and everything looks alright and connected but it runs like a dog if I tighten down that cap. It still does it to this day. The 87 is carbureted but has an electric pump in the tank and a mechanical pump on the block! -never figured that one out. Car was bought new in 86 with 3 miles on it so I know it came that way.
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post #5 of 15 (permalink) Old Nov 3rd, 06, 04:03 PM Thread Starter
 
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Re: q-jet problems

yeah it does have vacuum in the tank. tried taking it off and running it without it but the problem persists. i'm frustrated as hell now .
i can rev it up to 3000, wait a while until it starts shuddering and sputtering and whatnot, let it back down to idle, and as soon as it hits idle i can do it again and it still takes the same amount of time at 3000rpm until it starts sputterin again. if i don't let it hit idle before i rev it up again its stays in "sputter" mode until it hits idle.

like i said i'm pretty frustrated , so i'm done for the day...before i start throwing tools or worse lol
i'm gonna have a mechanic friend look at it sunday
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post #6 of 15 (permalink) Old Nov 4th, 06, 09:26 AM
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Re: q-jet problems

Vacuum advance problems? I'd view the marks with a timing light during a run.

If you a standard non-advancing light, you will have mark the harmonic balancer BTDC in 10* increments to keep viewing. Stock HEI's generally run 45*-55* of timing at part throttle cruise.

Another suggestion on fuel, once it starts to cut out, view the secondary rod well and see if fuel is present in the well. No fuel, delivery problem.

Spark plugs in good shape? Cheap investment. Autolites would be my choice.

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post #7 of 15 (permalink) Old Nov 4th, 06, 01:09 PM Thread Starter
 
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Re: q-jet problems

thanks for your suggestions...i'm definately second guessing myself that its a carb issue. I'm thinking that a sensor or something is telling the car to adjust the mixture or something. like i said i'm no mechanic. but my buddy is a mech and when i explained what was happening he told me it was probably a bad sensor and first one to check is the o2 sensor. I replaced that with no results. i pulled the codes off the ecm: codes 23 and 45 popped up. found that code 45 was oxygen sensor reading rich but still don't know what code 23 means. if i can't figure it out today he'll be over tomorrow to look at it.
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post #8 of 15 (permalink) Old Nov 4th, 06, 02:10 PM Thread Starter
 
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Re: q-jet problems

important update...i pulled the fuse to the ecm and the car ran great...a little rich but can fix that later. put the fuse back in and the problems returned...pulled it out and no problems.
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post #9 of 15 (permalink) Old Nov 6th, 06, 12:33 PM
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Re: q-jet problems

23 is MC (mixture control) solenoid (I think) grounded or open. That would do it! When you turn your key to the on position you should hear a clicking noise at the carb -you can also test them by sticking a small screwdriver down the center vent hole (car off!) and seeing if it is stuck in the down position -should be on a spring and you can easily push it down and feel the tension. Was a common problem on cars of that era. It has been a LONG number of years since I have worked on one of those!! It would probably cause the code 45 also. Sounds like you need a good mech. who can rebuild your carb for you. Those carbs require a little bit of cutting to get the plugs out of the idle mixture holes. They are a pretty straight forward Q-jet other than that MC solenoid. They do require some adjusting with a scan tool though if I remember right -like I said, it's been a while!
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post #10 of 15 (permalink) Old Nov 6th, 06, 03:13 PM
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Re: q-jet problems

my guess is check the mechanical advance weights.
they come into play at about the rpm your having problems at and I suspect a broken spring.
that if your sure the fuel system is good.
dont forget to check the sintered metal filter in the carb aswell!
some people dont realize there is one there.


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post #11 of 15 (permalink) Old Nov 7th, 06, 05:00 AM
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Re: q-jet problems

If it has codes or check engine light start with that first. Their is a few things that could happen with out codes but check that first . Auto Zone will check codes for free.
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post #12 of 15 (permalink) Old Nov 7th, 06, 08:54 AM
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Re: q-jet problems

Code 45 says the O2 picked up a rich condition -code 23 is your MC solenoid open or grounded. If your O2 were actually bad it would have thrown a code 13. Your O2 is fine -it's your carb.

Your MC solenoid controls how lean or rich your engine runs -it is either stuck down or there is an open or ground in the circuit -thus your carb is running rich and your O2 sensor picked that up. The MC solenoid sticking in the down position was a very common problem.

Here is a site that tells you what the codes are: http://www.totse.com/en/bad_ideas/sc.../gm_codes.html
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post #13 of 15 (permalink) Old Nov 8th, 06, 09:39 PM
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Re: q-jet problems

Before going too far, try this quick test.

Try pulling the hose off the EGR valve and see if it still happens. If the EGR valve goes bad, it can open too soon at part throttle and the engine will sputter.

The EGR pluggs with carbon inside the stem which is hollow and it causes it to open too far too soon at off idle. Pulling the vac hose to it will keep it shut, the engine should run cleaner but under load will want to knock due to no EGR circulation.
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post #14 of 15 (permalink) Old Nov 9th, 06, 07:40 PM Thread Starter
 
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Re: q-jet problems

thanks for all your help...the prev owner had the carb rebuilt a few month back and i'm guessing it wasn't done properly. talked to another rebuilder and he said he can fix it pretty cheap since he probably won't have to rebuild and replace everything. thanks again for all the help
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post #15 of 15 (permalink) Old Nov 10th, 06, 06:58 PM
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Re: q-jet problems

Didn't know camaros had carbs in 84. Surely it's not a rev limiter on the ECM kicking in. ECMs will do that when the car is in neutral just reving the motor. Not sure about the 84 ECM though.

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