1989 almost stock with automatic transmission. 54000 miles

if you look back at all my post, one will see that I have been working on an engine miss problem for almost six years

The miss is noticeable at idle, but It may be there at higher rpm, but I just Don’t notice it.

I also can’t tell if it’s only one cylinder or more the one. I can not determine a pattern, so I believe it’s random

i have tested, replaced and adjusted everything I have researched on the problem, or that someone has suggested. If you review my post, you will see all that I have done. I don’t want to bore everyone one with suggestions that have been discussed many times before.

As soon as the weather gets a little cooler, I am going to make one last effort to fix it. If I can’t, then I am done and car is gone. Car is in great shape otherwise. I can feel the miss while sitting in driver’s seat and it drives me crazy

I have broken down my approach to two areas. It’s either mechanical, meaning valve train, cam, timing chain, or electrical, meaning wiring. It cannot be sensors or adjustments. Been there, done all that.

Both heads were rebuilt and all that goes with that process. I had spare ecu and even swapped that out.

I need a positive procedure to determine which area the problem is in. In the old days, a shop could hook up a SUN engine analyzer and that would at least let me see on a scope how the electrical portion was working and if the problem was electrical, was it more then one cylinder. It would also show me if it was random or steady. If it all looked good then I would know it was in the mechanical side.

Question?
If I do a cylinder leak down test, and all shows good, can I eliminate mechanical, or could it still be on the mechanical side?

I am not a great mechanic, and maybe not even average, but I am always learning and I am not afraid to dig in to the engine.

Love car, but need help.

Mike

 

I am not going to go back and read all the posts, but have you swapped the distributor cap and maybe the distributor itself ? Way back when, the '65 I had experienced a high rpm miss. I replaced the distributor. It was worn. After replacement, it ran great.

Also, have you taken it out at night into a really dark place, with no lights, no lights in the car (cover the dash up, whatever) and looked for the blue glow of an electrical bleed ?

 

Im assuming the injectors have been replaced..when you time it is the mark steady or move around? Similar issue suspect the dist gear is fragged.

 

Everything mentioned replaced and or checked. Timing mark is steady. Three sets of new injectors, two new distributors, three sets of new plug wire, several sets of new plugs, changed plug gaps several times

 

I would do the leakdown test, yes.

 

How old is the Oxygen Sensor in your Corvette? I have seen them where you slow down in a hurry and driving conditions change radically very quickly the engine would stumble and take a second or two to regain composure. As they age they slow down and their signal is not as strong so it is best to replace them on occasion.

How accurately is the CTS displaying? An item we have seen before is where the CTS has failed and the computer sees that it is -30 degrees below when it is actually a nice late summer day and 85*. The Coolant Temperature Sensor is supposed to read the current accurate data to allow it to help control the fuel mixture. If the ECM sees -30* and it is really 85* your car will have it's electronic version of Choke "Activated" allowing it use more fuel to keep running since it is very cold according to the ECM.

Had it failed the other way it would make the car very hard to start or lets just more difficult to start without the alternate form of fuel enrichment. It would assume that you need zero help and in really cold weather that would be ugly.

Old Oxygen sensors over 2 years or 50,000 miles need to be replaced anyway. If your O2 is brand new then I would go to your CTS for the source of trouble. Just remember that your MAF system uses I believe two relays to control and perform the "Burn-offs" of the 1/10 mm wire. Always replace the relays when you replace the MAF.

Before going anywhere into the engine I would also try a can of B&G 44K Fuel Injection system cleaner. It would always smooth out my L98 idle for me. I strongly support the B&G product as it truly works and WILL make your (American) car work better. The German auto industry uses it here in the U.S. every day when people first complain about their idle. I have used dozens of their 44K additive in my families cars and have always been happy with it. My mechanics all love the stuff but never tell anyone they have it, they use it in a tool and charge $75 bucks more.

Good Luck getting that miss under control. Also make sure your knock sensor has not become more sensitive as it got older and started falsely triggering knocks which in 1988 meant all eight cylinders were retarded because of "Knock" that was detected.

Best regards,
Chris

P.S. Look for dry brittle wire on the intake manifold near the injector wires. The ground wires going to the injectors sometimes loose their insulation and will break or corrode on intake manifolds. Verify the engine is grounded well to the frame of the vehicle and that the ground straps are good.

 

It will eliminate some, but not all. When the engine is cycling, dynamics come into play (Mass, harmonics, momentum). You can image something moving at 10RPMs will behave differently at 3000RPM. A compression test will tell you your bottom end is sealed. Rings/Piston/Cylinder Walls/Valves/Heads are all capable of keeping the combustion chamber sealed. There is no harm in doing a compression test and you might find something.

Do you have scan data? Are you watching knock counts, BLMs, and if the information the computer is getting is inline with what you would expect?

 

I have replaced o2 several times with different brands Have replaced CTS several times.

The grounding idea is a good one. I think I will buy some grounding cables and try several from the engine to frame

i will also do compression check again when I do leak down test

thanks to all who are responding

 

Have you hooked a good logging scanner to it and looked through the results when it's acting up? It sounds like you replaced a lot of parts without diagnosing the problem. Not meant to be mean, we've all done it. What the sensors are seeing will tell you pretty much everything if you know what you're looking at.

 

As Vader suggested, do a leakdown to eliminate a mechanical problem. This will be a good indicator of the mechanical health of the engine. It won't help diagnose a broken valve spring, but that is usually accompanied with popping out the exhaust or intake.

 

..... What brand spark plugs and what are they gapped at ? Is this a hard miss or an intermittent stumble ? As the fuel control (O2S) vacillates from slightly rich to slightly lean our engines will stumble … the 80's EFI systems were very unsophisticated and the issue may be your perception and not the car … How does the car run otherwise ? …..

 

When I got a valve job on the heads, wouldn’t a broken spring had easily been notice? I do notice something does not seem quite correct while listening at the tail pipes. The random noise or tone I hear is there at idle and i just assumed it was because of the miss I have been trying to find. Would a broken spring be noticed at idle? Would I notice a problem at higher rpm? How about bent push rod? I do not notice any problem while driving. I only NOTICE it at idle.

Not sure what a broken spring would sound like.

I did do data log and sent it to well known person for tune. A few changes were made but nothing big, and nothing mentioned about finding anything not normal.

when I bought car it had 47000 miles on it and has had this problem from day one. I just figured it needed good tune up. The car burns no oil at all and starts right away. Engine does not shake standing next to it, all seems ok. Problem only NOTICED while sitting in driver seat at idle, and now I believe also while listening to exhaust at idle.

I’m betting doing a leak down test is really going to be a pain in the ***, but sounds like it has to be done

 

I have tried at least 5 different brands of spark plugs and different gaps over the years. Has made no difference

some times when hope runs out and you have tried everything that you know of to do,
It is very easy to JUST replace parts hoping you get lucky which usually doesn’t work.

sometmes I wonder if I’m finding a problem that isn’t really a problem, but then I remember I owned a Brand new 89 just like this one and it idled just find. I have also owned other high performance muscle cars over the years and remember how they idled.

 

So far response has been great. Looks like I need to try following things so far

ground straps
compression test
leak down test

i have avoided compression and leak down test because of difficulty of doing it, but no longer!! Have to fix car or it’s gone, and I will be honest if I sell it even though it will cost me

i would be willing to do another data log at idle if someone with a lot more knowledge then I do would like to look at it.

just for info, I use an ostrich II and can also burn chips if I need to. The problem was there before I installed ostrich

 

Have you used an inductive timing light and watched the firing patterns to determine which cylinder is misfiring ? (If it is an electrical misfire). Using a timing light lets you see if the pattern is regular, spotty, broken up etc.

 

I haven’t but will try it. I’m not sure if I have the ability to notice if it shows a miss fire. I think Im so far down in the dumps about this long term problem, I am just looking for reasons why something won’t work to find out what the issue is, but I will give it a go

thanks

 

Someone said check knock
sensor. Replaced long ago and data record show no problem

all sensors have been replaced at least once. The problem is not going to be any sensor. It could be wiring to/from sensor, but not sensor

 

This is why it is good to look at scan data... what does the ECM see? Are you seeing knock counts randomly all over the place?

 

Very few if any knock counts on data logging

checked if several times at various speeds and rpm

if I ever find problem, its probably going to be a simple repair, but my time is running out

 

And loads I hope.

Have you checked for play in the timing chain? I see it's low miles... but