Hi,
My father in law has a c4 with an L98 engine. It's a '90.
I've already fitted a hypertech cap and coil, new spark plugs and leads.
Also fitted new edelbrock fuel pump and sender. A new 02 sensor has also recently been fitted.
The idle on the car has never been great. And seems to get slightly lumpier when warmed up.
However just recently he has noticed a change in exhaust tone. And when under full throttle the car seems to be holding back.
I've ran a fuel pressure check and its within the limits of 34-39 psi.
Today I dropped the main cat off and started the car, the idle was slightly improved. (He said it had never idled this well).
However still sounded hesitate under throttle although this was in a parked position.
I shone torch through the main cat and could see it out the other side, does this mean it's ok??
I tried to boroscope up the the inside of downpipes to look at pre cats but could only get to the start of the internals.looks like a plastics square grid.
Before I go ahead and order new dowpipes do these symptoms sound like block or deteriorating cats. Either pre, main or both?
The injectors have been electrically tested and are ok, although the spray pattern has not been checked.
Any other suggestions as to what could be causing this?

 

Just ram something in there and ream them out.

 

What the pre cats?
I think it'll be pretty tough to ram something in there due to bends. Maybe I'll have to drop them off the manifold and ram something down that way.

 

There is a back pressure gauge that goes in the o2 hole to help diagnose the cats.

Gary

 

Is this a diy gauge to make up? Ours something I need to purchase?

 

The 02 sensor is before the pre cats. What benefit would there be measuring from here

 

If the idle is worse after warming up that indicates that the computer is having problems maintaining the idle during closed loop. There are two things that I would check. First would be the Idle Air Control (IAC) valve in the throttle body. Carbon buildup there can cause idle problems. Use a throttle body cleaner to flush out the interior of the IAC housing. (It wouldn't hurt to clean up the rest of the throttle body at the same time.)

The next item I'd check is the oxygen sensor and it's connections. You really need to use a program like WinALDL to see if the oxygen sensor data is cycling between rich and lean. (I don't know if any of the forum sponsors offer a USB-to-ALDL cable, but I've used this source before with excellent results. You get the cable and software on a CD for one fixed price.)

The CATS and pre-CATS are really mesh-type units (as opposed to the earlier GM pellet types). You really have to exam their surfaces carefully to ensure that raw fuel hasn't melted the substrate effectively blocking part of the exhaust flow. From what you've stated, it appears this may be part of your problem. Don't forget to check the mufflers for rusted, collapsed internals.

Flow testing the injectors will determine if any are leaking or have faulty (partially blocked) spray patterns. I recommend FIC and their services. They have sponsored the forum in the past, and Jon has posted many videos detailing injector problems.

You didn't mention how many miles your dad's 1990 Vette has. Carbon build up especially on the intake valve from bad oil seals can effect idle also. CRC Intake Valve Cleaner can remove this carbon build up as well as carbon deposits in the combustion chamber from worn rings. I follow the directions and inject it directly behind the throttle blades using the fresh air feed in the throttle body (that goes to the passenger side valve cover).

Hope this info helps resolve the idle issue.

 

I agree with giving the throttle body, plates and iac motor a good cleaning. It should be part of a tune up.

If the cats are starting to clog the gauge will give you a pressure reading from the cat being restricted.

Gary

 

the idle has settled down after removing the main cat. The car has covered 48000 miles. Thanks for all the other advice. Is it still worth me removing the pre cat pipes and gutting them to see if it makes a difference? Before ordering a y pipe without cats.
Thanks for the ideas, however I'm in the uk so my options for people and services are somewhat limited.
Could these also be linked to the hesitation problems as well? It really feels like it wants go but is choking itself, and he has stated the different exhaust tone recently? Which kinda pointed me to cats. I've been reading this forum quite a bit, but thought I'd post my own problem.
Thanks again

 

Quick pre cat check: Loosen the head pipes @ the manifolds a short noisy ride should give results.

 

sounds like a plan.
But only snag I can think of is I haven't disconnected the air pipe from the main cat as I don't know how to!!!
So that'll drag along the floor!!
How do I remove the air pipe from the side of main cat?

 

Be sur to put in new o2.

 

New 02 has already been fitted

 

Ok just keep it in mined in case you nuked it.

 

Is there a check I can do where I remove the 02 sensor and if exhaust gas comes out the cats are clogged/clogging

 

Pre cats are now off the car. Have to wait until tomorrow to drive it! It's very noisy!!!
Really
Hope this is the problem

 

Gutted the pre cats on my 90 the ol' butt dyno liked the change.

Had plans for headers later on so I modded the stock head pipe. Cut a "window" in the bottom side of each pre cat with a cut off wheel, gutted the cats, welded the cut out pieces back in place.

Good luck

 

I plan to cut the cats out totally and weld a flexi exhaust piece in its place.
Rather than have a hollowed out cat.

 

Update!!
I removed the exhaust system.
Cut open the pre cats and then ground back the housing of the cats
Ordered two stainless steel flexis to go in there place.
Bolted it on car!
WOW.
Sounds awesome and is no longer holding back.
Next thing to cure is I feel that it's idles slightly low. (What could be the cause)
And now on kick down the revs run Upto 6k redline before changing. Is this normal?
It's never revved that high before or us because of the restricted pre cat being removed?

 

In your discussions I got the impression that it was an exhaust item causing you that problem. I spent days myself on the same thing on a German car.
I have a 1988 L-98 and it ran great for a while and then stopped running so well and I did the standard tune up and to no avail. I spent a few weeks trying "Deep Cleaning" the fuel injection system and other things and in fact it was getting worse as time went on.
It turned out that my Exhaust Gas Re-circulation system had frozen open and it completely plugged itself with a huge amount of carbon. The solenoid that controls the vacuum through that area had ceased to function which led to serious vacuum leaks. After taking off the entire intake system and cleaning it out with a sand blaster I reinstalled everything and wow, what a difference. No more air leaks messing with the idle controls.And best of all I go a fully functional EGR system so now I pass the Virginia Emissions Tests with ease! When I updated the exhaust system on my car I replaced it with a single Hi-Flow cat system. I love my old cars but I try to keep their exhaust as clean as I can, within reason.
The EGR problems seem fairly common on the L-98 and with a bit of preventative maintenance you can avoid the hassles.