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
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?
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
Ford 22# injectors work and cost $309 for 8 or the GM injectors used
on the LS1 work for $336.
I would hazard that it is an injector issue.
Response is embedded:
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!
I'd suggest not buying more GM Multecs. (see
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.
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.
Had the same problem on my 1985 with 55 k miles, dealer changed the PCV
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.
pull the plugs and look for a fouled one. replace that injector. The dealer
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.
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
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
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
my .02 cents
My 90 is now running great. Here's the final rundown, in chronological
- 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
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
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
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