1986 corvette. Aluminum heads. 80k miles. My idle likes to bounce between 6 and 7 rpms. Does the same thing in park or drive. It’s not just sitting on the border between 6 and 7 rpms because I can feel the engine not running right. It's like it has a stumble that goes up and down. It never drops below 6 though and I’ve never had it stall or anything like that. I’ve done everything I can think of to smooth the idle out and it still runs the same so I’m at a complete loss. No codes either. It drives great with plenty of power but the idle bothers me because I know there must be some way to fix it.

A video is attached

things I’ve replaced or fixed:

-cleaned throttle body and put new gaskets and put brand new IAC valve in
-set tps to .54
-set timing
-new spark plugs
-new fuel pump, fuel pressure regulator, fuel filter, and fuel pump pulsator
-cleaned MAF
-new air filter
-new oxygen sensor

 

Have you checked the resistance of each injector?

If they all look within normal range you could have a mechanic hook up a cleaning fluid to the fuel rail and clean them on the vehicle.

 

Since you have had the intake apart to replace the items below the plenum I would be looking for a vacuum leak.
My 86 TPI never idled smooth. I bought the car with the engine apart and got it running well enough to get it to the dyno shop. There were a few vacuum leaks fixed at the shop that were at the evap canister, nowhere close to the intake. These rubber hoses are very old and the whole system should probably be replaced but that is a big job. Not technical but time consuming. Even after the dyno shop got the engine running well the idle was never quite right until I installed the edelbrock pro-flo intake. After that, no idle issues at all. So I know there were leaks somewhere in the TPI. My troubleshooting vacuum leaks is obviously fallible because I never did find the issue with my own TPI. With that said, I did have a good bit if success with a home made smoke machine. Google search will find you the instructions. Finding vacuum leaks is certainly no fun. I would start with plugging all vacuum connections on the intake with caps except the fuel pressure regulator and I would run a short direct hose to that. With everything on the intake closed off you can eliminate leaks at the intake and then connect one system at a time to see where that takes you. My hvac hoses behind the radio had been chewed by mice and I had to correct all of that.

 

I'll second Vac. leak.

 

I'd have dumped the pulsator.

Check that the fuel pressure is right and holds. Next thing is to see what the IAC count is on a scanner when everything but the motor is off and is at operating temp and closed loop.

 

Hello, You could try some Lucas fuel injector cleaner it might help. I use it all the time and it keeps my engine running smooth! Good luck!

Aà

 

Uh Oh....

ELIXIRS!

 

I don't and that also keeps my engine running smoothly. I lowered my emissions by dancing around a fire at midnight with a chicken on my head.

 

Hello, I didn't understand your message but it sounds like you nearly hurt yourself. Lucas fuel injector gets put in your gas tank not directly into your engine and its safe to use. Good luck!

 

That's about all it is...b/c it has to be. That you poured some potion into your tank, and "haven't had any problems", is proof of nothing. It's evidence of nothing. It's very similar to:

-and then claiming (possibly rightfully) that you haven't had any problems. The chicken dancing isn't the reason for success. Like Aklim, I use gasoline from a basic gas station....and it "keeps my engine running smooth", on my ~200k mile 'Vette....but that ain't gonna help the OP if I tell him that. He has an actual problem that needs an actual diagnosis...followed by a repair. Checking for vacuum leaks (after an intake tear down) is probably the BEST advice given, so far. Diagnosing injectors is also good advice. You might think about giving THIS A READ...





.

 

Gasoline is safe to use. Does exactly the same thing. Separate the rube from his/her money.

 

Set timing to what? Have you ensured that the indicator isn't "walking" because it is old and separating? That could cause erroneous setting.

What have you used to clean the MAF? Have you done any modifications to the MAF?

Why did you replace the IAC? Was there something wrong? Was it from a reputable source?

 

I have heard this before....

Craig Agiro you have suggested something that "certain" members will Never agree with. That is their Right as Fellow Citizens of this Great Nation and I will defend their right.

Lucas is a great product and is made for fleet use. I use a quart of the Lucas Oil Stabilizer with every oil change in the family fleet. I am also a BIG believer in B&G 44K Fuel Injection Cleaner as it is used by hundreds of mechanics a day (and it works...). I know it helps and makes my L98 engine run smoother after using it. There will always be "doubters", I have to be "convinced" but will trust quality products. If I "believe" it but you "don't" that is your right and that is what makes our country so great.

If you start looking at your car try and start with your vacuum system. The EGR is operated by a Vacuum Solenoid mounted on the thermostat housing. A PWM signal is sent from the ECM that controls the opening and closing of the solenoid. When the vacuum stop big problems follow as the system gets carbon all over inside of the small vacuum openings in the block. I had to spend a lot of time cleaning out hard carbon and then I reassembled it with new parts. Do not try and disable it or remove the EGR without modifying the code used in your Corvette.

Triple check for vacuum leaks and verify the MAF for working properly. If there is any leakage post-MAF and the Throttle body intake the idle will never smooth out. You might try some thick grease on the throttle shafts where they meet the throttle body as that is often a vacuum leak.

 

Idle speed variation is normal. The low resolution (x 100 rpm) of the digi-dash just doesn't capture much of the variation.

Here's an example log of an auto trans car idling (in gear) to help illustrate the point:

https://datazap.me/u/tequilaboy/ae-t...rk=11241-11455

This car is modified with blower, heads, cam and custom tuning, but idle rpm will never be dead flat. Note: the 650 rpm desired idle speed shown here has been increased by 100 rpm as compared to stock (typically 550 rpm with warm/hot coolant)

The variation in rpm is largely due to the mixture variation which is due to the action of the integrator in closed loop along with spark advance variation. Rpm will rise as the mixture is leaned out. Flatter spark advance tuning would help somewhat here. Keep in mind there is some transport delay between what's happening in the cylinder and the wb02 reaction. This can be as much as 400-500 ms and can be observed during fuel cut events.

Interesting to note that the idle is typically offset above the target by about 50 rpm and is varying +/- 50 rpm or more all of the time so 100 rpm peak to peak swings aren't unusual. With the large dead band (50 rpm), the IAC isn't doing much correction.

At least the OP's rpm is not dropping low relative to target which would be an indication of a misfire. I assume the video is with the car idling in park/neutral with 600 rpm target (550 plus 50 rpm offset).

If someone has log data of a stock or nearly stock car to share, post it up.

 

Ctmccloskey has a potion for every imaginable symptom.


Well...that ain't true and you know it...so why did you post that horse ****? I use some products for a specific purpose. We use a BG product at work for a very specific purpose. What we DON'T do, at work (or at home) is arbitrarily pour unknown potions into various orifices of our fleet vehicles for no good reason. We diagnose and repair actual problems.

 

You ever asked they won't publish certified laboratory results and depend on unscientific testimonials?

Maybe check if there is a leak. You can't adjust minimum idle if it is leaking since it will be pegged at 0 even if you close the butterflies. A scanner is under $100 and will save aggravation in the long run. We don't even know if the reading is good or bad.

 

A scanner would be the direction to follow at this point. Having a scanner is probably the best way to find problems but not everyone has access to one. I am glad I bought mine. Basically start at the beginning resetting the throttle body parts should get the idle to calm down.

Tom400CFI, I am glad to hear that you use B&G Products. Getting chemicals in the right orifices is an important task, not a problem I have ever had....

aklim have you ever seen what it costs a company to get outside verification testing of a product? It is way more than the marketing budget could or would afford. I am not sure I would trust it if they had. I rely more on the experiences of the mechanics that I know in the area.

There are many products on the market which actually do what they claim. No having said that there are a lot of products which are not able to deliver on their promises. Consumer Reports is a widely accepted company but their opinions are questionable. I don't believe a word they print. They are so pro Asian it smells funny to a guy who likes German vehicles....

Personal experience with good or great results is what makes me believe in a product. I don't recommend anything that I don't use myself. I I will not use it I will also share why I choose not to use the product.

Have a great day!

 

Its about $100 and it can save a lot of aggravation. Definitely cheaper than paying for a scan and it is nice going into the shop knowing what needs to be done.

Wish I had enough people as understanding as you. We'd never have to prove anything. Sorry but I think they have the money after fleecing all the suckers into buying the product. Unless, of course, the test would NOT prove the point, then it would be better to rely on testimonials instead of a failed test. Any of the mechanics you speak of have been able to do a double blind test or whatever the automotive standard is? Trump testified Hydroxy chloroquine works. If you believe that it does, with testimonials without proof, I think we call it faith.

 

Yep....use BG (not B&G) at work for a very specific reason. Know what that is? To fix an erratic idle? No. To improve hard starting? No. To clean injectors? No. Because...."Never had any issues yet!"? No. Are we just guessing and throwing a potion at whatever, for who-knows-what-reason? No.

We one product, in our bulk diesel tank, that prevents the fuel from gelling at cold temps. This product allows us to continue to use summer blend diesel through the winter w/o gelling or waxing. The summer blend is cheaper than winter blend, even after adding the BG. That is why we use it. SO....as good as it works for that specific purpose, you'll not see me telling people on these here forums to go dumping various crap in their various orifices to fix un-diagnosed issues with their cars. Know what I mean?


.

 

Here in Northern Virginia the local Advance Auto stores will scan your car for free. ($0.00)

Hello again Tom,
Thanks for clarifying the B&G versus the BG part.
I use Lucas fuel additives but not for freeze protection but for Ethanol protection in my fuel. It cleans the fuel system and keeps the ethanol from hurting my engine
Are you familiar with Justice Brothers products? I met a local sales rep who did a demonstration of his product Justice Brothers Oil Conditioner and he did it again in front of my fellow MIT Engineers at Westinghouse. This was not a group of "believers" either.

Justice Brothers had a Demonstration Rig that incorporated a drive motor and a bearing making contact with another smooth bearing rolling at whatever speed they wanted. They used plain old Dinosaur juice engine oil and demonstrated how the bearing was operating fine. Then they increased the bearing load by 10% and somewhere early (25-40%) the plain motor oils seized up and scored the bearing. They installed new bearing and did the test with Synthetic oil which allowed for slightly higher loading before it too seized. Then they did the same test using plain motor oil with their JB oil Additive in it and they were able to go to real high on the loading since their additive was doing it's job. They put 10 times the loading force on that bearing and all it did was slow down but never seized or was damaged in any way. Due to skeptics in the room he did it twice explaining every detail as he went. The Sales rep then took a previously damaged bearing back on the machine and the mixture of JB and Motor oil and let it run for about 10-15 minutes and all the damage and scarring was gone and the bearing smooth and rolling along again. This test is pretty convincing and proves that their product actually does what they claim it does.

Not every product is Bogus.
Not every business is based on products that don't work.
Not every person is "bad" at heart and wants to screw everybody over.

My colleagues were quite impressed after witnessing this demonstration. Several of them bought products right there on the spot.

I guess my point is that there are many "good products" out there and there are even more "BAD products". They need to be evaluated individually and not all lumped together. Teflon in your Engine is a bad idea but they sold boat loads of that stuff even after DuPont came out and publicly announced that the Teflon would potentially damage your engine. It does, the by product of Teflon inside a Internal Combustion Engine is an acid that will even eat Stainless Steel.

I have lots of experience with using BG 44K Fuel Injector Treatment, Does it make the engine run smoother? Yes. Does it make the fuel system cleaner? (can't see inside) Does it improve hard starting? yes. How do I know this to be a fact? Experience with the product. I have used it in cars diagnosed with faulty fuel Injectors only to have it smooth out the idle in 20 miles driving with 44K in my fuel tank. $20 saved me from having to spend $200 on a set of injectors.

When somebody comes to the Forum with idle issues I frequently recommend a less costly fix over buying a scanner or replacing the injectors right away. I am an old school mechanic that believes in fixing things instead of throwing parts at a problem. I also like to figure out why the problem occurred in the first place.

Anyway, you have your opinions and that is your right. You don't have the right to try to "humiliate" people who believe "differently" than you do. This means If I suggest BG products I shouldn't have to count on getting poop for my opinions.
In our great Country, The United States we are allowed to have our "own" opinion.

If someone suggest something that is "wrong" then you can and should correct them to help them both.
If someone does something you don't agree with does not give you the right to discredit them or make fun of them.
If you really don't know much about someone I would hesitate to criticize them or their knowledge.

I am a Careful consumer and I don't buy thing because they "Might" work, I buy them and recommend them because I "KNOW" they work.