1st of all, HI to the forum. New here but glad to be among Vette freaks. Now to the problem, short story. 91 275 hp. Daily driver. 110,000 miles. Super clean. A couple of weeks ago, stop and go traffic and had a really big miss. Got progressive worse. Changed dist cap, rotor, plugs, wires, fuel pump, fuel filter, MAP sensor, checked fuel pressure (42/40 and holding - 38 running), checked regulator vacuum, EGR vacuum, all injectors clicking, plenium clean, no gas or water in oil, no bubbles in the radiator, no vacuum leaks. After all of that, car runs ok (not great) when cold. When warm, at idle, misses like crazy; when cruising at constant speed, it surges.
I think I'm down to some weirdly plugged injectors, or some plugged up cats.
Looking for some imput, someone who has had this problem and the fix, or just some really educated guesses.
Thanks for the help
Subman

 

Check the Oxygen sensor, hook up a fuel pressure gauge at the rail and shut the car off and see if pressure holds or drops to zero. If pressure holds for several moments, the injectors should be ok, if not theyre toast.

If the cats glow red or there is excess heat in the footwell, they are probably toast too.

 

vader - thanks for the reply. Check the fuel pressure and it held steady for over 1 hour. Did not think of heat on the floorboards. Local muffler shop will test cats for free. Will try to get there over weekend.
Subman

 

What did you gap the plugs too?
also do a compression check


[Modified by bill mcdonald, 11:56 PM 7/1/2003]

 

I'm thinking the coil may be crapping out. I see that you changed about everything, but the coil wasn't mentioned.

 

bill & ittlfly - thanks for the input. gap on plugs .035. did not check coil. Will do so tomorrow along with the cats. If I need injectors, a friend can get Pythons in a box from Python really cheap. Any info on them would be appreciated. (BillMcDonald - I worked with a Bill Mc Donald years ago in a LA printing co. any relations?)
Subman

 

had the same problem on my 91. did all that other crap, didnt help. so i talked to tpis, they told me it sound like a fuel problem. then they told me to check the ohlms on the injectors. the injectors are supposed to be between 14-16 ohlms. i had 3 that were at 7ohlms cold. changed those 3 and same problem still. so i checked the others hot and found 3 more bad. so i ended up replacing all of them. runs perfect now. so warm up the car and check all the injectors for ohlms make sure they are between 14-16 if they arent, any of them, change them all, not just one. advance auto has very good oem injectors for $58 a piece. dont go with accel not very good. bosch has very good injectors also. but if your running stock motor stay with the 22lbs injectors no more. the reason why you car is surging and missing is that when the computer is taking over its seeing a rich mixture so it pulls fuel away from the injectors to try and compensate causing a lean skip. if you put you car on a scanner you could see exactly what computer was doing. in the obd 1 software on your car there is part of the diagnostics called block learn. it is supposed to be right around 128. as my car warmed it started to fall down to 110 which meant the computer is pulling fuel back as far as it can. as soon as i replaced the injectors it stayed at 127 all the time. also the 91 vettes were known for fuel injectors going bad.one more thing. if an injector is lower than 14 its throwing more fuel that it should, if its higher that 16 its throwing less. an injector that is out of range cannto be cleaned because the mechanical part of the injector has failed. and if you do have so in range i wouldnt risk leaving them in.

 

Subman,
As mentioned, check the coil, and those injectors.
If your into pulling the plugs, you might want to go for a .045 gap.

A buddy of mine put his plugs in his 90 at .035 and said it ran really bad. I told I thought gap was suppose to be .045 on the later L-98's.
He said, go ahead and pull the plugs if that is what you think, so I did, and at .045 it ran great.
I have ran my 90 at .045, now I am at .055 with a hypertech coil and MSD on it.
This is something you can play with once you get it worked out.


BTW, that was not me in LA.

 

mos90&bill- thanks for the info - checked the coil, it's ok. had the cats checked, they are right in the middle (good is 2 lbs pressure, bad is 7, mine are 4 lbs left precat and main cat, 5 on the right pre cat, replaced the coolant sending unit, checked the egr valve (again) and the egr sending unit, cylinders are between 142 and 155 on the compression test, checked the park/neutral switch (somehow it is tied into the egr valve solinoid and the idle)So I guess it's down to the injectors. Have a friend that can get me Python injectors really resonably (will find out how reasonable tomorrow). Again, thanks for the help.
Subman

 

142 and 155?
Did you guys spray some oil in there for the compression test?

this is a 91 right? It shouild have a 10.25:1 compression ratio.

Mine is rebuilt at 10:1, it hits 190 on compression dry.

 

91 have 9.6/1 comp ratio. should be around 160 max.. so he is close.

 

So the 91's were dropped from the 90's 10.25:1?

 

Thanks for the replys - - 1st - compression test was dry, but your questions raised a question, so I started lookin for the specs on the compression, factory pistons, etc. Found nothing on pistons, the "book" says the compression should be "the lowest reading should be no more than 70% of the highest reading and nothing less than 100lbs" Okay then....onward. I did find something I thought was weird. In re-checking the EGR valve, there is no vacuum from the solenoid when cold. None when warmed up either. Nor when accelerating. I went through all of the tests for the solenoid (pulling vacuum, electrical, including the "in drive and accelerate to 2000 rpm" everthing checked out. The EGR holds 7 in. of vacuum fine. The solenoid also hooks up to the TB (idle control??). Am gong to replace injectors this weekend. While I'm in there, should I just change the EGR solenoid and the valve just to be safe?
One last thing - - I don't exactly what month this 91 was built, but I have alot of 92 placements on parts. When I look at pictures of where things are supposed to be on the 91, like the MAP sensor, they aren't there. Look at pictures of a 92 and, oh, there it is.
Thanks again for the help and ideas.
Subman

 

Subman, a 91 and a 92 are very different engines. That is very odd to me.
Do you have a digital camera that you can take some pics with? This way we can see what is going on.

I would do the EGR for sure if you are "going in" to much trouble to come back and re-do it.

the solenoid is mounted by the thermostat housing right? (It is suppose to be) It should be easy to get too, rather the plenum is on or off. Well, you might want to break that thermostat housing bolt loose (rust etc) while the plenum is loose if you need to get it out later.

 

the egr will not have any vacuum. when i had mine on the scanner it will tell you the % of vacuum the egr is working. i had 0 vacuum at all times. you should be able to disconnect the egr if you wanted to and have no problems running.

 

but it cant hurt to change it

 

bill, about the comp ratio. i have been researching it. ive been reading that the 90 had 9.5/1. and i saw that the 91 had 10/1 but some are saying 91 had 9.6/1 so i dont know what is correct yet..

 

I ordered the helms manual, it came with a small sheet of updates, one of them was that the 90 had a set of pistons special to the 90. 10.25:1 compression and something about a barrel shape.
It made mention that these pistons were not interchangable to other L-98's

It was discussed n here before, but it is not comming up in the search engine.

 

mos90 & Bill-
Thanks again for the help. Will try to borrow digital this weekend so you can see the engine (may I could wash it and post pictures of it clean). Tested ohms tonite cyl. 1,6,8 measured 5.3 ohms; 2,3 - 17.2;4 - 17.3; 5,7 - 17.1.
So, if your info is correct, and we have no reason to doubt, 3 injectors are over fueling and 5 injectors are running lean. Python injector deal fell apart so now I need to find 8 new ones by Sat.
Bill - double checked gap on sticker - said .035 but will give .045 a try once I get this injector thing worked out.
Assuming that all goes well with injectors, changes to plug gap?, with all the new stuff, this puppy should run out.
Again, thanks for the help. Will keep posting updates.
Subman

 

Found some Bosch injectors from fiveomotorsports locally. Should know about availability in the morning. A couple of questions were raised in earlier regarding the engine, parts placement and stuff on my 91. So I looked in the book, noted some differences from the pictures to mine and decided to check the VIN. The first 5 numbers are no biggy, 1 g 1 yy. Now comes the weird part. Body types listed are; 07-hatchback; 02-hatchback; 67-convertible; 3-convertible. mine is 23 (?) and is a hatchback. Next is engine code-8-L83 (1984) or L98 (1995 up). Mine is 8. Year code for 91 is M. ECM is located over cruise control left side firewall. Evap canister is right rear 1/4 panel, coolant res. right firewall (my 88 was here too) MAP sensor mounted to plenium, no MAF, EGR solinoid mounted under plenium at the TB. The air intake duct looks more like 92 w/o the MAF. At this point most of the rest of it looks like the photos of the 91 and earlier.
Subman
Subman