Hey guys, just picked up an 85 last night. Driving it home, i made some observations (Got it for 2500 from the fiance's dad).

First, what would cause a rough idle on a FI engine when the plugs and wires are all new? Throttle pisition sensor? isn't there some kind of idle sensor as well? Or should i start by replacing the distributor? It's going to get new wires next spring btw (i think he just put the regular stockers on there but i'll get something a little more HP)

Second, rattle trap. Is that normal for a vette with 73k on it? Seems that the top and the dash rattle the most. I tried to tighten down the bolts that hold the top on, but the tool that the car came with is basically a ratchet with a torx on it and the ratchet was broken. What size torx is that? I can't seem to get the torx tool off of the ratchet.

Third, not sure if this is a problem or just a trait of the steering. Car seemed to be all over the road. When i turn the steering wheel from left to right real fast, it doesn't feel like anything is loose. Steering is very tight and VERY responsive. I can turn the wheel like 1 degree and the car wants to turn, unlike any other cars i've driven. Any input would be great, plus anything else i need to look for as well.

Thanks,
Mike

 

Could be a lot of things causing a rough idle, but replacing the distributor probably won't fix it. The first thing you should do is get a Factory Service Manual (FSM) before you start throwing money and parts at it. Helm sells these.

Is it setting any codes (check engine light illuminated)?

First thing I would do is Ohm the injectors, that is what caused the rough idle on my 89. Take each connector off and use a meter that can measure resistance, put one lead on each tang. They should all be relatively close in the readings, like 16-17 ohms. If you have some that are lower ( I had one at 9 and one 13) they are bad and need to be replaced. If you replace them, do them all at the same time, not piecemeal.

You could also check the fuel pressure, they sell gauges for this that connect to the schrader valve. My 89 runs about 39 psi at idle with everything connected.

You won't get more HP on a stock car with 'better' wires unless there is something wrong with the ones on the car now. If the wires & plugs are new as you say, I would take them off one at a time and Ohm those as well looking for one with a higher resistance. Make sure they are not burned through anywhere and are connected tightly to the plugs & the cap. Replace the wire and move on to the next one. Then remove the plugs, clean them, make sure the gaps are set correctly and the porcelin isn't cracked and put them back (I recommend using anti-sieze even though you have iron heads). I would also take the cap off and inspect the terminals on the inside. If it is the original cap, I would change it.

Others will chime in here I am sure, but I would start with the above.

I believe the suspension on the 85 is pretty stiff, so it doesn't surprise me you think it is a rattletrap. I would just hunt down the noises when they start bugging you enough. Has the heater core ever been replaced (part #0000000001)? If you take the dash cover off to look for noises consider replacing it because it will go on a vette that old. I did mine myself, the part is cheap ($50) but the labor at a dealership would be a killer. Do a search here on the heater core and you will see what I mean.

My Vette was a little squirelly on rutted asphalt, but a Camber Brace from R&D took care of that! (Link on the left, they are a supporting sponsor)

 

my 86 also feels pretty 'all over the road' and i've been told that it's because the tires are so wide they react more to ruts.

 

Thanks for the input. I probably won't do anything to it right away as i only have a few weeks left this year to drive it around before i put it in a garage for the winter. first things first though, i'll be taking it in for an oil change right away as my fiance's dad said the last time he changed the oil was last fall, and he's only put about 1000 miles on it since then.
Funny you mention the heater core thing, as i remembered something else from last night, other then the factory radio doesn't work. I couldn't move the temperature bar from cold to hot. Not sure why. Something is hanging it up, any ideas?

 

My 86' has very tight steering. Any slight motion of the wheel causes the car to start turning. This took some getting used to, but I like it. It does make other older cars feel like the wheel is connected to the wheels with a wet noodle.

As for the idle. There are a lot of things that will cause a rough idle in an L98. This is NOT a complete list, but it could be:

- Bad O2 sensor
- Bad Idle Air Control valve (IAC)
- Bad Throttle Position Senson (TPS)
- Bad Mass Air Flow sensor (MAF)
- Carbon buildup in the throttle body
- Bad spark plugs, wires
- Bad Distributor rotor or cap
- Bad coil
- Incorrect ignition timing

 

....about the car being all over the road. It's probably not in the steering at all - that sounds solid and tight. Check for play in the rear wheel bearings. That's a common replacement item in C4's. Too much play in worn out bearings will cause the car to move around on bumpy roads. In essence your getting some steering (rear) from play in those bearings. Jack up the rear, a side at at time, and grab the tire on opposite sides and rock it back and forth and feel for play in the bearings. I don't know how much is too much, and maybe someone else with more experience can fill in on that spec.

 

So how do i go about testing the sensors? I don't want to just start randomly replacing sensors when they are anywhere from $20 to $50.

 

Get yourself a can of SeaFoam, it's a carbon remover and fuel system cleaner...most guys, including myself swear by it. You can follow the directions....or do it my way. My way is, add 1/2 can to fuel tank, and 1/2 through the brake booster vacumn line....it'll smoke like hell, but that's what you want, and the smoking will quit in a few minutes....it may cause the check engine light to come on (throw a code) don't worry about it if it does, just disconnect the battery for a few minutes and it will clear the code (most likely a misted 02). BTW, I keep the RPMs up some while adding it.

An 02 sensor, or a MAF sensor normally throw a code, whereas an IAC (idle control sensor) doesn't. My bets on the IAC, although that's just a guess, other than the items previously mentioned.....it could also be a vacumn leak.

 

i figured that the MAF or O2 would throw a code, but wasn't sure if the others would or not. I'm wondering if the TPS isn't bad. It idles so low when it's sitting at say a stoplight in drive that it almost wants to die, but it doesn't. I know that my fiance's dad said that sometimes it would die and then not start for a few min. And he said that every time he's tried to sell it before it wouldn't start at all. I'm assuming that these problems are probably all related somehow. Hope that helps a little more with the diagnosis. What does putting the carbon cleaner in the brake booster line do? Thanks guys.

 

Quick quiestion guys. When your trying to turn your wheel when backing out of a parking place, does the steering wheel feel hard and feels like there is no power steering or is it suppose to turn with ease? I was thinking because of the wide front tires that it makes it harder to turn.

 

A wealth of info here on low idle speed and how to correct it.


http://forums.corvetteforum.com/show....php?t=1861263

 

I would check the spaghetti vacuum lines. Theyre made of brittle plastic and theyve been enjoying hot/cold cycles for the past 22 years. Also the intake manifold bolts have a tendency to loosen up over time, and this would also cause a rough idle. The engine basically gets extra air that the MAF sensor doesnt register because its entering the engine after it. You can check for vacuum leaks with an unlit propane torch...when you hit an area with a leak, the engine will idle up. Just roam around the engine compartment with the tip of the unlit torch, following seams, gaskets, vac lines, etc.

As far as the wandering, these cars are extremely finicky when it comes to alignment. I had the same issue when I first bought mine, and a new set of BFG CompZR's and a complete 4 wheel alignment made all the difference in the world.

The best advice I could give for the squeaks/rattles/groans...get a REAL LOUD RADIO!!!

Good luck, and welcome to the family!

 

Funny you should mention that as the factory radio doesn't work!

 

Doubt very much it's your TPS, from what you just said "Dad said" I'd be running a fuel pressure test on it right away...before anything else. And if you're thinking I'm thinking injectors you're right....as long as it cranks. A vacumn leak can cause idle problems......but I don't recall a vacumn leak keeping it from starting.

 

what should the fuel pressure be at and how do i test it? also, how do i test injectors? What ohms should the be at?

 

You'll need a fuel pressure guage(about $40.) at Autozone, and it's a tool you'll use often. At the end of the fuel rail, passenger side there is something that looks like a tire valve...it's called the Schrader valve, take the cap off and attach the fuel pressure guage, run the bleed off line into a container, turn the key to the ON position,listen for the pump to prime...usually about 2 seconds....note the reading (not sure about the pressure on an 85) no matter, turn the key OFF, and watch how long it takes for the pressure to drop, and how far from the original reading. My 87 hits about 40 PSI. Someone will chime in about an 85. What we're looking for is signs of leaky injector(s). Now according to my 87 manual, the Ohms should read above 10 Ohms, mine read something like 16 if memory serves (cold). What we're looking for here is the overhaul health of the injector coils.....so all injectors should read within +/- .5 Hope this helps getting you started.
Go to Ebay and see if you can find the HELMs or Factory Service manuals...year specific for your vette, mine cost around $100. for both sets, best tool you'll ever buy.

 

Test the injectors and check the fuel pressure and get back with us. You may want to take the throttle body off and take the IAC out of it. Clean both with throttle body cleaner and a toothbrush. On the IAC, make sure the tip is no futher than 1 1/8" from the flange when reinstalling. The seafoam in the brake booster line is a good suggestion also. Check and make sure all the plugs are getting fire.

Is the rough idle mostly with a cold engine, hot engine, or both?

 

You must be rich - new wires, and yer gonna' get more new wires next spring???

Pretty cool - the ol' Fully Automatic Assault Wallet approach.

Pick me up a set?



Rough idle? As in 'loping' - up and down revs?

Vacuum leak.

 

no, as in idling so low it feels like it's going to die, and it feels like it's missing on a cylinder, but as soon as i get on the gas it wakes up. And to answer the other question, it does it whether it's cold or it's hot. As far as the money thing, from what i've seen about what guys have done to thier vettes and after seeing the prices..... the 1200 i want to spend on it in the next year is NOTHING compared to what those guys have done, let alone the price of just the car.

 

Two things you can also do, lift the hood while it's dark out...look for any arcing, and you can actually ohm check the plug wires too. They should ohm at about 5000 Ohms per foot, kinda twist them as you're checking them, and remember they're all different lenghts, so take that into consideration.