Flat Spot

[spro2395]

Hello
I have been the owner of an 85 Corvette for 6 fun days it's gold with 78000 miles. So far I like it very much but it seems to have a small flat spot when I step on the gas. It's fine once I get past it also the idle speed goes up and down a little is this normal for the TPI ? Thanks Joe

 

[Schrade]

Welcome onto the boards there!

You have a vacuum leak - loping idle. Textbook indicator UH HUH. See FIRST quote below...

Flat spot? Hmm... Even if you could tell us what gear, and rpm's you were at when it happens, I think no one could tell a thing.

Closed loop (fully warmed up) idle quality tells more than ANYthing. Any codes? Search the boards for paper clip trick. Get a multimeter, FSM from eBay, and then you'll need to do a fuel pressure test. Search my posts for EXACT procedure; search youtube for test vids (reference only, not exact procedure). There is SO much importance in proper fuel pressure...

No one got the proper technique for vacuum leak detection... :confused

FIRST; block the pedal, or block the throttle linkage. YOU HAVE TO DO THIS TO GET A STEADY IDLE , around 1,250 rpms, or a little higher. If you don't, you can fish a propane hose, or WD40, or Krylon, or hair spray, or pee, anything, until you need a sweater in Hades, but if the motor is lopin' already from a vacuum leak, you ain't gonna' find any vacuum leak, unless it's so big that Little Jack Horner can stick his thumb in it and pull out a pineapple NOPE.

AFTER you get a steady idle, THEN, you can spray starting fluid everywhere - that's the best detector, because it's vapor pressure is very high - a LITTLE at a time, until you get the SURGE. Follow the vacuum lines to the EVAP cannister too. You might even have to follow the EVAP lines from the EVAP cannister to the gas tank too, but that vac leak will show up as a DTC 32 EGR fault, and won't normally show as loping idle, unless the EVAP cannister has been by-passed (been there, saw that uh-huh).

If you get a surge in a tight spot, and can't tell exactly where it is, light a book of matches, blow out the match heads while they're still burnin', and feed the smoke to the tight spot.

Fear not the starting fluid; you cannot put the can down fast enough and light the matches fast enough to catch the starting fluid, because the vapor pressure is so high.

Bookmark this post YUP.

Wrench Wizard OUT!

Welcome to the boards!

You have at least one vacuum leak, and another in vac hose that supplies vacuum to EVAP cannister.

This is a SPECIAL vacuum leak. Standard vac leak test does not apply, since EVAP purge vacuum is applied ONLY when the ECM tells the EGR solenoid to purge the EGR AND the EVAP.

First, READ (don't DO it yet) this standard vac leak test, and make sure you understand it.

If the hole in the EVAP vacuum hose is not big enough to see, you'll have to modify the vac leak test, since vacuum is NOT continuous to the EVAP.

To modify the test for the EVAP vac circuit, pull the hose that goes to the EVAP cannister FROM the intake. Pull it where it feeds INTO the intake, and connect it to a CONTINUOUS vacuum source (remember, the normal port is NOT continuous, rather it is open ONLY at speed when the ECM open the EGR solenoid).

Once you have it 'T'ed into a CONTINUOUS vacuum source (there's several CONTINUOUS ports, some on the throttle body, some on the intake - just make sure it's 'T'ed in tightly), THEN do the leak test described above.

You WILL have to feed the starter spray everywhere, as described, and ESPECIALLY along the modded vac line FROM THE MODDED 'T', TO THE EVAP CANNISTER, SINCE THAT'S WHERE AT LEAST ONE LEAK IS.

If you can get it right, you're the man. I know only one other guy who did the REGULAR test right, and found his leak...

EDIT:
THIS IS ANOTHER POST TO BOOKMARK YUP!

 

[spro2395]

The flat spot is right off idle in park or taking off from a stop sign I don't think I noticed it when your moving. The car seems to have OK power and looks like it's been well cared for . It has a new K&N air filter but I don't know about the filter or the pump.

 

[spro2395]

I drove it to work yesterday ran fine on the way in but the service engine soon light came on. On the way home and it chugged under a load at 40 mph in od in traffic.

 

[spro2395]

How do I check for codes? Do I need a code reader ?

 

[Schrade]

How do I check for codes? Do I need a code reader ?

See post #2, 4rth paragraph, reference to 'paper clip trick'.

You have a vacuum leak. Do you understand the vacuum leak test as posted?

Open loop cars (cold motor) ALL run great. Again, see post #2, 4rth paragraph, reference to closed loop idle quality.

 

[spro2395]

Hello
Can you tell me which ports are A and B for the paper clip trick ? I can't get my big head under there to make out the letters.

 

[Schrade]

From here....

The vacuum leak (which you definitely have) will not show as a code, except maybe DTC32 EGR. You may have others, and you definitely have the leak.

 

[spro2395]

Thanks for your help Schrade! I have a code 42 and I'm still looking for the vacuum leak there are a lot more vacuum lines on the car than I thought. I am going to clear the code to see if it comes back. How long will the codes be stored?

 

[Schrade]

If it came up once, it will come up again.

They'll store forever, unless they are cleared from memory. Clearing them from memory does NOT make the problem go away, as many people wish. Out of sight, out of mind...:eyerole

If the fault is repaired, and the code is NOT cleared, the code will go away after x number of drive closed loop cycles without the [repaired] problem being detected again. The key is REPAIRED.

Again, clearing the code does NOT constitute a repair.

 

[spro2395]

Yes I understand. The guy I bought the car from told me he found and repaired a code 42 I want to see if he cleared the code or not. I'm going to email him to find out what he had done to fix the problem.

 

[Schrade]

Yes I understand. The guy I bought the car from told me he found and repaired a code 42 I want to see if he cleared the code or not. I'm going to email him to find out what he had done to fix the problem.

Maybe skip the email. You can't count on other peoples' words for ANYTHING. Only when the words are on paper, waiting for cash that stands behind the words being truthful, does that stuff matter.

My .02...