I have had my '93 Corvette LT1 manual for a couple months now and it seems to lag and stutter, and accelerate very slowly when trying to accelerate until I reach about 3000-3500 rpm. This doesn't happen if I punch it, but only during normal acceleration for the most part. My car is a stick and typically idles about 1000 rpm. The previous owner said it has new opti-spark, new spark plugs and new plug wires as well as a Thermo Master Power Chip.

There is also an issue with the exhaust popping when releasing the gas, and often around the same RPMs as the lag. I'm not sure if this is related or not.

Also possibly related, or possibly not, the car seems to lose alot of power above 5000 ish RPM. Is this typical, and should I be shifting about there under hard acceleration? What is the Redline?

This issue has been a problem mainly for the forced 1-4 shifting where when I get into 4th it starts lagging and I have to shift to 2nd or 3rd. Also when I'm in 6th and going below say 70-75 it lags and stutters if heading up any incline or trying to increase speed a bit.

Any help would be appreciated.

Thanks,

Richard

 

My first suspects would be low fuel pressure, bad fuel pump or clogged fuel filter.

 

Your Post brings back "Bad" Memories.. My 95 did the exact same thing.

This is what happened to me, before I had a chance to resolve the Hesitation problem my Head gaskets cut loose and started blowing bubbles in my recovery tank, also the tank level was eratic. the second and final symptoms were high head temps (Analog gauge) and finally.. Steem out the pipes.

My gaskets were leaking in several place's, heads were warped .005" and my intake guides were worn to the Max limit.

After cleaning up this mess, I can't be happier the car pulls a grade in 6th and runs like a bat out of hell.

I hope your problem is something different, but a Block test would not be a bad idea, at-least you could rule this possibility out..

Good luck..

 

I would hope it is a fuel filter as opposed to warped heads and a blown gasket. What should be my first step to diagnose this? How do I do a block test? I have some basic car skills and I'm not at all afraid to get my hands dirty and for the most part wouldn't hesitate try things I've never done before versus taking the car in, both for money savings and learning experience, but I am an amateur, and self taught when it comes to working on cars.

Also, driving my car yesterday I pinpointed more where the power falls off and it is right at 4500 RPM. The car really noticeably quits pulling as much at 4500. Is this normal?

Also, I do have a compression tester that I've never used. I picked it up for like a buck at a garage sale a few years ago, and never used it if that will help with the block test.

With regard to the blown gasket, I haven't noticed any fluctuation in the overflow tank, or bubbling. Also I've never seen the car overheat. Typically runs around 180. Has gotten as high as 230 in the summer in traffic, but always drops back down when the fan kicks in. These are all from the digital gauge as my analog gauges don't seem to be working correctly.

Thanks,

Richard

 

A leaking Head Gasket would be your worse case Scenario, hopefully this is not the case..

- Yes change your filter, and I would do a fuel pressure test.

- A "Block test" would tell you if there are a mixture of combustible gas's mixed in with your Anti Freeze. (Leaking Head gasket)

- Power dropping off could be insufficient fuel flow, it could also be an ignition problem, or a Mechanical problem.

- A comprssion test is a good idea.

- Although you run 180°-230° which is pretty normal the engine may have been heat stressed in the past.

- I would get the Analog gauge fixed.

So you have quite a few Jobs to do, I would start with the Fuel system. A Fuel pressure gauge that could be taped to the windshield will allow you to observe the Fuel pressure when the power drops off. I would do this before changing the filter. If the pressure drops off under a load then change the filter, and see what this does. Bottom line.. Maximize trouble shooting while minimizing parts swapping.

There are a bunch of guys here to help you along the way..

 

That is a great break down. I'll stop by Oreilly's today and borrow a block tester, and get a new fuel filter while I'm there. I need to change my oil soon anyway, so I'll check for anything weird there too. If nothing there helps I'll go ahead and get a fuel pressure tester and check that and compression. How hard is a compression test to do? I've never pulled the spark plugs. Are they easy to get to? I'll report back once I get some more info.

Thanks for the help,

Richard

 

Some of the plugs are a bear to get to. Might as well replace them if you're going to pull them anyway. Use anti-sieze on the threads.

Fuel filter is a bit of a PITA to change, so do your pressure test first.

 

Ok, then I'll check the fuel pressure first and do the block test. Having some trouble locating a place to loan me a tester so I might just buy one. Carquest has them for $33, and that would be worth it to ease my mind since now I'm a little worried in case it is a blown gasket or cracked block or something. What should my fuel pressure be, or should I just be looking that it doesn't drop off when the car is stuttering or losing power? I'll try and find the numbers in my service manual when I get home, but I am still trying to get familiar with it so it takes longer than it should.

Thanks,

Richard

 

So I did the pressure test. Was about 41ish before turning on the car. Then around 35 at idle. When I accelerated and was below 3000 rpm the needle was all over jumping like crazy...seemed to coincide with the stuttering, but didn't ever just plain drop off...jumped maybe +/- 10 psi centered around 35-40. When it really lagged, the needle jumped even more. I don't know if the fuel pressure is causing the lagging or the lagging is causing the fuel pressure to jump. Any ideas? Is it normal for fuel pressure to jump up and down, or should it remain relatively constant?

Next up I'm going to do the block test. I'll report back when I do that.

Thanks,

Richard

 

Change the filter and report back

 

Did the block test, fluid stayed blue so thats good. Does that mean it definitely isn't a head gasket/cracked block? Looks like changing the fuel filter is the next step.

Thanks,

Richard

 

Rich,

Fuel pressure should be constant with a slight variance in pressure between idle, and high power. You need to do a pressure regulator test, and inspect the Vacuum line going to the regulator for evidence of fuel inside of the hose it-self. Also.. make sure that the Vacuum line is routed correctly, and tight.

This could be a bad regulator. Another test that might be helpfull; disconnect the Vacuum line while under high RPM's and observe the fuel pressure gauge for fluctuation.

Another Video..

http://www.2carpros.com/articles/how...-and-regulator

I would hold off on the Block test for the time being.

 

Ok, I'll check that stuff out on Sunday. Taking my wife to the cabin this weekend and need to pack up. I'll get back at it when we get home. I'll post what I find.

Thanks,

Richard

 

No.. not 100% a gasket can leak Chamber to Chamber, A Compression test would test for this problem..

Hold off for now, and focus on the fuel system, you need to complete one task at a time.

Hey if you decide to jump on the fuel filter make sure there isn't a Hot Water Heater close by..

 

Thanks for the water heater tip, unfortunately I don't have a garage, so all my work is done outside.

I'll test the regulator as you pointed out then try the fuel filter if I can get to it Sunday.

Thanks,

Richard

 

Hi rjschwar,

Also suggest checking the ICM (Ignition (coil) control module). It helps control the spark timing. Make sure it is tight (14 inch lbs +/- but check FSM), and that there is heat sink paste (NOT diaelectric grease -- heat sink paste available at Radioshack).

the ICM is small electronic module mounted to the front of the passenger side head, likely to an aluminum heat sink and then to the coil bracket.

When I replaced mine, low end hessistation went away (also car started again, as mine died completely).

It is atttached by just two small screws (in lbs of torque), and can be removed and tested at Autozone or some other car parts places).

If replacing, ONLY use ACDelco, other brands have been reported to cause problems.

 

I checked the fuel regulator. When I pull the vacuum line both at idle and at higher RPM, the pressure jumps about 10psi which sounds like what is supposed to happen. Didn't check the ICM yet. I noticed that pressure was constant when not turned on but at idle the needle moved +/-2 or 3 psi quickly. Is this normal? It isn't nearly as bad as when I was cruising during the stuttering.

I finally have been figuring out my FSM a bit in terms of where the hell stuff is, and I did test the pressure as well with the fuel pump 2 fuse removed and it went up to around 48psi as the manual specified for a couple seconds before settling around 40. When I turn on and give it some gas it goes up for a second a couple psi then settles around 38.

Another thing I remembered, I have been getting intermittent Check Engine Lights, and the code is for the EGR (32). According to the FSM, my issue could be ECM open at idle, but all my engine lights have been after I'm at speed for a while. I haven't gotten around to fixing it because I was lead to believe this isn't a big deal. Might this be part of the problem?

Also, I don't think I mentioned it but since we are talking fuel system, the previous owner put an after market throttle body on. I've never really done anything w/ a throttle body, but there are 1 or 2 ports that look like a hose should connect to them that have nothing connected. Is this normal? If not I can try and post a picture of it.

Suggested next steps?

Thanks,

Richard

 

Looks like you had some time to work on her.. I'm not real clear on your first Paragraph, do you still have bouncing fuel pressure W/Fuel regulator Vacuum line disconnected? (But not as bad)

There is a section in the FSM that describes an EGR valve test, at some point you will need to check this, but for now I would keep it on the list.

I'm guessing the two open ports on the TB are about 3/8 in size? and down low, if so those were for engine coolant, and were bypassed.

 

When I disconnected the vacuum, the psi increased about 10 psi and held steady...no jumping.

Yes, the TB ports sound like the coolant...good to know.

I saw the EGR test but didn't have all the right tools for it if i'm not mistaken.

Thanks,

richard

 

Do you have a vacuum gauge that you can connect to the Regulator's Vacuum line. Test Vacuum at Idle, and a slight rev. It should be steady at idle 17-20in/HG and drop smoothly when you rev.

Look at this procedure..

http://autospeed.com/cms/title_Using...3/article.html