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help - loss of power in '85

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Old 06-17-2018, 05:53 PM
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sb66
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Default help - loss of power in '85

hey everyone

I went to take a short run today with the '85 (4 + 3 ). and it started right up ready to go - then as I started up the street - it seemed to be sounding much more rough then before - almost heavy and more thumpy(I believe that's a technical term used in some lesser circles of knowledge and the English language). I drove for a while hoping it was due to sitting for about a month (my apologies to the Vette gods - back injury). as the drive went on it did not improve - if I tried to give it extra gas it tried but did not come close to feeling like it wanted to - very sluggish, heavy and yes thumpy. So I stopped turned around and headed home.

I did some searching but as soon as I mention power loss - it blames the L98. I was looking forward to a couple of days off to enjoy the ride -

Temp gauge was normal -

any thoughts opinions, etc would be greatly appreciated

Sean
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Old 06-17-2018, 06:13 PM
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Spark-air-gas

Sounds like you have spark and air so I'd check your fuel pressure. Lack of pressure can make a car feel sluggish. Put a gauge on your fuel rail and see what ti says.
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Old 06-17-2018, 06:50 PM
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no codes???
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Old 06-17-2018, 07:07 PM
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I did not see a code - they will show up visibly on the dash - correct? (I know probably a moronic question - I have not had any before)

I will see what gauges I have -

thanks
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Old 06-17-2018, 09:28 PM
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no error code lights up on the dash
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Old 06-17-2018, 09:49 PM
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sb66 are you willing to get your hands on a few basic tools like the fuel pressure gauge suggested? it will be something simple and basic most likely. we can guide you through a series of elimination.

if your fp is ok, my bet is on the icm. but u have to stay methodical and do this stuff in steady succession.

VT
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Old 06-18-2018, 06:59 AM
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thanks - Yes I am going to see what I have - If I need to I will get a fuel gauge tester. Is there any specific kind I should look for ? or is there a "general" tester for this?

"icm" ? sorry still learning stuff.

thanks - really appreciate everyone's willingness to help guide me through this

Sean
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Old 06-18-2018, 07:03 AM
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You can rent a fuel pressure tester at autozone or the other store like that. Just tell them the which car you're testing.
You could have some bad tune up parts, sounds like its misfiring.
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Old 06-18-2018, 09:54 AM
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https://www.summitracing.com/parts/a...model/corvette

icm?

not buying anything yet - just making sure I understand stuff

thanks
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Old 06-18-2018, 10:04 AM
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Old 06-18-2018, 10:20 AM
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Originally Posted by sb66 View Post
https://www.summitracing.com/parts/a...model/corvette

icm?

not buying anything yet - just making sure I understand stuff

thanks
Yep, that's an ICM. In my experience, the ICM either works, or doesn't - but I've heard of ones that fail partially. Assuming fuel pressure checks out, and there isn't a vacuum leak, then I would start with the ICM.

When the car loses power, does it feel choppy, like you are having a misfire, or is it smooth, and just doesn't pull? If its missing, ICM, plugs, and wires would be my first items to check. If it feels smooth, then I would check knock sensor/knock sensor connector, replace the gas in the tank (I siphoned all but 1 gallon out, then went and got gas), then check the ICM.

It is especially important to replace the gas if it is older than 6 months, in my case the car would knock, the knock sensor would pick it up, and timing would be pulled which resulted in low power.
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Old 06-18-2018, 10:21 AM
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That would work for testing fuel pressure. You can also borrow one from an auto parts store if you don't want to shell out fifty bucks.
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Old 06-18-2018, 10:26 AM
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Basic tuneup, plugs, wire, cap, rotor. Check the fuel pressure, then a compression or preferably a leakdown test.
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Old 06-18-2018, 01:13 PM
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Originally Posted by TheBlaster9001 View Post
Yep, that's an ICM. In my experience, the ICM either works, or doesn't - but I've heard of ones that fail partially. Assuming fuel pressure checks out, and there isn't a vacuum leak, then I would start with the ICM.

When the car loses power, does it feel choppy, like you are having a misfire, or is it smooth, and just doesn't pull? If its missing, ICM, plugs, and wires would be my first items to check. If it feels smooth, then I would check knock sensor/knock sensor connector, replace the gas in the tank (I siphoned all but 1 gallon out, then went and got gas), then check the ICM.

It is especially important to replace the gas if it is older than 6 months, in my case the car would knock, the knock sensor would pick it up, and timing would be pulled which resulted in low power.
the car is definitely not smooth - feels like it might be lacking compression - so I guess that would be missing? not very good with the diagnostic side of things - I am sure this should and would be easier for some -

I will do a visual check - and probably end up down the road buying a fuel pressure tester - sounds like it will be useful through the lifetime of the car
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Old 06-18-2018, 01:19 PM
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Originally Posted by sb66 View Post
the car is definitely not smooth - feels like it might be lacking compression - so I guess that would be missing? not very good with the diagnostic side of things - I am sure this should and would be easier for some -

I will do a visual check - and probably end up down the road buying a fuel pressure tester - sounds like it will be useful through the lifetime of the car
Ok, if it feels rough, I'd bet on a misfire. You can rule if the issue it gas-related by doing a fuel pressure test, if pressure holds during the miss, you can move on to suspecting spark. Since it feels rough, I'd first replace plugs and wires (remember to gap plugs before you install them, ask me how I know). Then if the issue persists, go for a new distributor rotor, cap, and coil. Both of these tasks are rather easy to do. If that doesn't fix it, then the ICM. Should it still miss, then you may have a bigger issue - we can speculate after the easy stuff is checked.
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Old 06-18-2018, 01:29 PM
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thanks - really appreciate all the help
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Old 06-18-2018, 02:20 PM
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had something similar when I had a fuse went bad (didn't burn out but looked like it almost did) for my MAF module. I thought the MAF was the issue but luckily the fuse just needed to be replaced. Forget which it is off the top of my head but maybe take a look at your fuses. ICM going bad is also another good place to start, as is fuel pressure (rent the pressure gauge at Autozone, you can return for a full refund).
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Old 06-18-2018, 03:30 PM
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Mine had similar episode not long ago due to a spider leg stuck to my MAF sensor inside...
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Old 06-18-2018, 07:23 PM
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fuel pressure - 43 key to on position
40 at idle, and increased when I pulled throttle cable

those numbers seem ok?

the only thing I noticed was a slight puddling of some type of fluid by the thermostat housing - but I think that has leaked slightly for a while - so that really is not anything new - not necessarily good but not new

thanks

Sean
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Old 06-18-2018, 07:44 PM
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Originally Posted by sb66 View Post
fuel pressure - 43 key to on position
40 at idle, and increased when I pulled throttle cable

those numbers seem ok?
Yep you are good. Anything in the 40s at idle is spec.
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