Need advice on 90 Vette poor idling when hot

My 90 convertible is completely stock. Recently it had been acting up where it idles rough when it warms up. It has around 60K miles on it, and I use it as a daily driver in a local commute (no highway miles).
My local mechanic did a tune-up (replaced spark plugs, wires, etc.) but that didn't help. The local Corvette shop blamed the EGR valve and replaced that, along with the gas filter (which had never been replaced as far as I know). The car seemed a bit better for a couple of days, but went back to idling rough. I took it on a trip with about 200 highway miles (Jersey shore) and by the time I got back in the 90 degree heat, the car was barely running; still no codes in the ECM even though it stalled on me twice.
Both places now say I have a bad injector. The mechanic says that he won't be able to tell which injector is bad with the old "pull the plug wire" trick because the computer compensates immediately.
A set of 8 is an expensive proposition, especially for GM parts (over $100 each) and I'm not 100% certain that will correct the problem. I've got almost a grand into this and I'm now looking at another grand.
The Vette shop said that the injectors could be pulled and tested. I've also heard that the dealer may have the necessary equipment to pressurize and blow out the injectors. I don't know the cost, and I wonder if it's worth replacing just one anyway. Are L98 injectors likely to start going bad at only 60K easy miles? (I have an 85 Eldorado with throttle body injection; the car has 150K on it and the same injectors are still working fine.)
Any advice? Test first or just replace? GM parts or aftermarket? Something else completely?
Thanks tbone
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try a new idle air controller servo
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Just for kicks get a can of Sea-Foam from Autozone or similar and run it through. May not make an immediate difference with the injectors but for $6 it's worth a try. Best distribution for cleaning the injectors is to suck it in with the PCV tube. Follow the instructions and don't avoid doing it all over in a day or two as the first treatment can loosen up a lot of stuff that will come out on the next try.
Ford 22# injectors work and cost $309 for 8 or the GM injectors used on the LS1 work for $336. http://www.fiveomotorsport.com/Injector_SetsTPI.asp#caddy
--
Dad
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I would hazard that it is an injector issue. Response is embedded:
tbone wrote:

Been there, done that

Not completely true since this vintage of L98 uses 'bank firing' where all the left injectors fire, then the right ones fire. Pulling the connector from a good injector will make the engine run much rougher--but that won't tell you much because the 'bad' ones are also supplying a 'lean' amount of fuel and pulling one of those will also make it run rough.
Save yourself some bucks:
If you don't have a good multimeter, take a couple of twenty-dollar bills and wander down to Harbor Freight or Radio Shack. Buy a mid-priced digital multi-meter -- something that's accurate and repeatable from zero to twenty ohms. (If you get it from Harbor Freight, the Chinese battery will soon die so buy a spare battery for the meter--but, get the battery elsewhere.) With the car cold, pull the injector wires, one at a time and measure the resistance of the injector coils. (Make sure you are getting good contact between the probes and the injector terminals.) Should be 16.5 ohms give or take an ohm. (Don't forget to consider--subtract out, the resistance of the meter leads -- probably 0.5 ohm or so.) The low resistance injectors have shorted turns (making for a lean situation on that cylinder -- run this way long enough and you may do real damage.)
Now, just for grins, put the connectors back on and drive the car. When it starts to run rough, get back home and immediately measure the resistances again. The ones that dropped have a progressive problem ready to leave you parked where you don't want to be. (With bank firing, when that progressive short happens, it electrically 'swamps' the injector drivers for the entire bank of four cylinders and things go to hell in a handbasket.) This second (hot) resistance check pretty well tells you that the issue is injectors --- PERIOD!

Ouch!
I'd suggest not buying more GM Multecs. (see below)

Replace injectors in sets that have been flow checked and matched. You should be able to find good refurb & matched sets for less than $ 500. Find a source known for good cleaning, testing and matching (within a couple of percent for total flow.)
Are L98 injectors

Most of the 88-89-90 injector issues are the result of shorted coils in the Multec injector. Although the valve design is great (looks like it was 'stolen' from GM's marine and locomotive diesel experience), the coils are cooled by fuel. In the early 90s, GM issued a bulletin to *not* clean these injectors with OTC injector cleaner. Also, this is one of the reasons that the L98 is restricted to 10% Ethanol. As we've screwed with fuel additives and gone on the Ethanol kick, these injectors have failed with increasing frequency--mileage driven since 2003 seems to be the driving factor.
Chevy had a good idea with this injector but their crystal ball didn't include what the Fed-EPA, the Congress and the Calif. greenies would do to fuel. There's been a p/n change but that doesn't guarantee that the new coil insulation is any better with the fuels we now have to contend with.

Some guys have reverted to the earlier Rochester injector which has been very durable (not many complaints from the 86-87 crowd.) Other guys have gone with Ford injectors. I think most of those are Bosch or Bosch copies. I chose a Denso injector and sacrificed top end for idle.
If you confirm that it is injectors then post here for injector recommendations. There will be plenty!
Consider whether you want good balance between cylinders at idle or increased fuel flow at WOT or whatever.

Hope this helps
-- pj
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PJ wrote:

One last thing to check before popping for new injectors is the fuel pressure when the engine is warm. Might be the fuel pump ... that caused a similar rough/stalling at warm idle problem on my ' 96.
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It's a low-tech suggestion, but how about checking the spark plugs?
That's what I'd do first. Look for consistency and general condition. If one is fouled or burnt, check the injectors. If they're all fouled or burnt then look at sensor issues.
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valve and said it was fixed ($250) to find this problem, I didn't get mare than a 1/4 mile and the same rough idle. I told the dealer to check the injectors in the first place because I had one plug that looked like it was running to rich. Went right back to the dealer then checked the injectors and found the cylinder #1 that had the rich running plug to have leak by. Then they asked if I wanted them to be changed for only $1600. I purchased a new set from a speed shop, used the same pressure rating as std. and 3 hours later the car ran better than new. Checking the resistance values of the injectors will only tell you if the coil is bad but will not show a fault if the injector is leaking. It cost $300 for new injectors well worth it. Bob T. 85,76
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tbone wrote:

should have checked the block learn and integrator (long & short term fuel delivery ) Also the egr passage could have carbon build-up causing the pintle to hang open. this will happen only after you reach "cruising speed" and then return to an idle.
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gratt via CarKB.com wrote:

No pintles on these injectors -- different design. -- pj
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Thanks for all the advice and pointers. Until I checked around online, I was not aware that a full set of flow-tested injectors could be had for under $800.
In the meantime, the mechanic said he checked the resistance on all injectors and found that six of the 8 are out of spec - three are high and three are low.
I am now planning to buy a set of Bosch injectors for my L98 - thanks Dad! They're only $308 for D5B stock replacements.
Thanks to all tbone
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tbone wrote:

Good move! -- pj
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some excellent advice above.....
i had a similar issue back in the early 90's where my GM car would also run rough after warming up and once or twice even stalled on me when i did a turn!
after going thru some basics like plugs and filters and checking injector firing using the old "listening with a long screw driver to base of injector" trick :-) found that some of the bolts on my intake and exhaust manifolds were slightly loose.
as far as i knew, it was as if the car's ECM was masking the issue by supplying more fuel to the engine when it was cold so there was no indication.
checking simple things first can sometimes save you a couple bucks...
my .02 cents
Harry

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My 90 is now running great. Here's the final rundown, in chronological order:
- tuneup; plugs, wires, etc probably was needed anyway; don't recall having done that - EGR valve was probably partly to blame (rough idle when hot) - fuel filter seemed clogged when shown to me; had never been replaced AFAIK - injectors out of spec resistance
It appears the injectors were the last problem in the chain. The new Bosch injector set only cost around $300, plus labor. I do think the EGR valve was partly to blame, and replacing the fuel filter probably helped too.
So now my 90 runs like new! Thanks to everyone who had suggestions! I wanted to post a followup in case someone else experiences this problem.
tbone
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