ksch994462

This is admittedly a shot in the dark. My '90 convertible with L98 engine is in beautiful condition; however, it idles rough, and feels like its jerking a little while I drive it, which makes the driving experience a lot less enjoyable. I have done the tuneup, including plugs, wires, dist cap and rotor. It only has 66k miles on it. It doesn't throw any error codes and the dealer service dept just says that some of the corvettes just idle rough. I don't want to start replacing perfectly good components looking for some good luck. What would the real knowledgeable corvette guys do in my situation? I really appreciate any suggestions you can make.

Churchkey

Get rid of the factory multitech injectors & replace them with flowed Bosch 111 injectors.

My 90 runs A+ after the swap.

Contact Jon at fuel injection connection. There should be a sticky about injectors.

Found the link

http://forums.corvetteforum.com/c4-t...questions.html

Joe C

while you're at it, clean the throttle-body...

esham

65Z01

Thanks for the link on those Bosch IIIs...

BADDUCK

It may be injectors but I am surprised the dealer didn't try to sell you new or at least check them. Before you rush out and buy new injectors, check the ohm readings (hot and cold). Also check fuel pressure. I had the same symptoms as you 3 years ago, and it turned out not to be the injectors. I am still running the originals in my 1989.

ksch994462

Thanks for the advice. I just returned from the dealer service dept & the guy I'm working with agrees. He describes it as a "hard miss," and if it was an electronic problem, it should register an error code, check engine light. He thinks it could be the gear at the end of the distributor shaft that's worn and it may be drifting in and out of time. Tomorrow, they're installing a new heater core (about $350) and they'll pull the distributor to check the gear.

65Z01

Wow, $350 for a heater core ain't too shabby...

BADDUCK

real troubleshooting is the way to go!

ddahlgren

Timing droping in and out will not create a miss just the performance will be sluggish on all 8 cylinders.. I am not a believer.. I wish I lived closer because for 350 I would gladly have them do the heater core and blend door stuff on my 91.. that is a steal so they certainly must be honest or looking to fill the service bays..

SunCr

Check #7 for a leaking head gasket; ie, check compression and complete a leakdown test or simply look at the plug and compare it to #5 or see if there's rust on the threads.

WW7

I had a miss in my 89 and it turned out to be the inside of the junk(Fram) gas filter had come loose and was blocking the gas flow. I also had a hole in my egr valve diaphram that made it miss and would make it jerk while driving. You can check the diaphram by spraying carb cleaner or TB cleaner at the bottom of the egr valve while the car is running , if the idle changes its the diaphram..If your filter hasn't been changed in a while do that also...I also agree with the others , check your injectors with a meter, they should be around 16 ohms both hot and cold..Good luck....WW

Joe C

resistance doesn't check spray patterns and flow rates. these are other factors to consider. injector filters, and fuel deposits on the internal passages can effect spray patterns and performance. i can see where one injector, even though it passes the resistance test, doesn't flow sufficiently, thus causing a weak cylinder. a couple of bad injectors, and all of a sudden, rough idle...

i'm not saying that's the problem, i'm saying just because an injector passes the electrical resistance test, doesn't mean it's operating at 100%. it's not a black or white thing.

the following is from a Fuel Injector Performance Testing & Restoration site:

A 5 Micron blockage at the injector's pintal area can result in a 25% blockage of Fuel Flow. Just 10% will begin showing as drivability problems. Oxygen sensors can only sense the overall lean/rich result
of a fuel system and at the present, the ECU has no way of detecting
or correcting a single Injector's flow problem.
Lean or Rich, correction is achieved by the ECU adjusting Injector
pulse-width or flow ratio to all cylinders. As the ECU struggles to
command a balance, the prolonged incorrect fuel volumes, result in lower overall performance...

a 25 year old car with original multi-tech's - i'd consider replacing them whether or not they pass the resistance test...

WW7

Your Absolutely correct , but Multec injectors in 89-91 are known to have problems with the coils shorting out, and this should be one of the first things to check with these years when your having these drivability problems.Personally, with the problems they have had with these injectors I would suggest replacing them even if they are still working...But thats just me, some guys have had no problems..WW

Joe C

my 90 has what i'm thinking, the original multi-tech's (53K miles on the clock). the car has no starting issues, gets decent gas mileage (actual 23.9 mpg highway), has a somewhat of a smooth idle, etc, but from a performance standpoint, it's a slug! the resistance test is good, but i'm replacing them with boschIII's (as soon as i get off my lazy a$$) -

Pwnage1337

First thing I would do is drive to the dealership and slap whoever told you that they are supposed to idle rough, he was just being a lazy ****

WW7

I agree , if a car idles rough theres something wrong, period. I bet when it was new it didn't idle rough, or it would of been back to the dealer getting its $40,000 azz fixed.. ...WW

coupeguy2001

Do yourself a favor, and try pushing all the wires onto the spark plugs until yoiu either feel them click onto the plugs or you hear a faint "click". Try it also on the distributor cap as well.
In high school, I did a tune up on my girlfriend's bug, and then she had what you are describing. Her father went out and pushed the plug wires onto the plugs better, and the problem went away.
He said "I thought when you work on a car, it was supposed to get better."
Never forgot that.

WW7

Ouch..I bet that hurt....WW

l98tpi

[QUOTE=coupeguy2001;1573215371]Do yourself a favor, and try pushing all the wires onto the spark plugs until yoiu either feel them click onto the plugs or you hear a faint "click". Try it also on the distributor cap as well.
QUOTE]


On my stock 91 6spd when the car would get warmed up it would have a slight miss inconsistently. I accidentally found it was on the #6 cylinder plug wire at the plug would not stay connected to plug. Replaced the plug wire and the other 7 as well and never had the problem again.