I'm fairly convinced the problem with the '88 is the MAF. I tried CRC cleaner and it didn't help. I can see the side of the MAF wire towards the incoming air is dark and the side towards the engine is clean / shiny. I tried CRC on a Q-tip and got it cleaner but motor still runs bad. The Q-tip turned red so I'm guessing the K&N was over-oiled and it coated the wire. Although the filter hasn't been oiled for several years.

Anyone tried this one:
http://www.am-autoparts.com/Chevy/Ca...FUMV7Aod1jQAwQ

They don't say the manufacturer but it comes with a 10 year warranty.

Thanks

 

why would you suspect the MAF as the source of the misfiring?

there are a thousand other things that can cause this as well....and they all will set a code of some sort.
have you pulled the codes?
MAFs are rarely the problem. They are self cleaning, like a good oven, in the exact same way.

Relays, wire harness, tired, dirty injectors, all are better candidates than the MAF sensor...
Dirty secondary ignition, bad wires, loose contacts in the cap. Bad module, no heat sink grease....on and on and on....

Please post your evidence, other than the wire "looks" dirty. That's thin.

 

I didn't mention misfiring in post #1.

This is the thread I have on the problem:
https://www.corvetteforum.com/forums...-to-check.html

 

OK, gotcha...

the d/c maf and improved performance is very tempting to blame the maf BUT...

the MAF is simply sending a set of values, (numbers) to the ECM to digest when many other numbers are added to the equation.

If the MAF were at fault, the ECM would quickly ID that and code it...like instantly.

SO....
what that indicates to me, is that one of the OTHER sets of numbers in the AFR equation is at fault and when you d/c the MAF and force open loop, the ECM is running everything off the MEMCAL pak which is the pre-packaged set of numbers for operation in case of emergency, like major sensor or component failure.

Fuel injectors.

If the air portion of the equation is good, and the ECM knows what's good and what ain;t, then it must be the other major part of the AFR, the fuel delivery system. At the AGE and history of (storage)this car, I can assure you that those stock injectors are NOT spraying atomized fuel....they are DRIPPING wet fuel droplet's into the intake which accounts for the crappy performance and consistent poor performance. When in OL everything changes and everything is a LOT more forgiving because its a pre-packaged set of values with a large window of operation, not as precise as real time data that CL offers, even with the ancient computing speeds of the C4 computers. Go to OL and everything gets a very wide margin for error, the expected AFR probably drops to 11:1 instead of the going rate if 14.7:1 when things are right. I know my cruising est mpg drops about 5 mpg or more, if the ECM goes to OL while hot and cruising. That's a pretty good size window.

Your fuel pressure leak down is a bit quick. That says the pump is getting loose inside.
It should get 40+ psi at Key ON, no crank.It should also hold that for more than a few minutes. Idle should show 38-40 steady as a rock PSI and it should hold close to 40 for a good hour then drop a few lbs each hour over the next 8 or so until its down to 20....and that should take all nite to go away. A good, tight system should show some residual fuel pressure in the morning after being run the day before..

A worn pump may not be able to keep up with demand and 40 psi at key ON, does not mean it can keep 40 psi at 3000 rpm under load....old/loose pumps allow pressures to fall behind as RPM increase and that will appear as mild popping or what sounds like a back-fire when you apply throttle...its lack of fuel. Low voltage or low fuel pressure on demand will produce this "back-fire" when throttle is applied at higher loads in the higher rpm range.

You also need to replace the cap & rotor with BRASS post. The HEI system will generate up to 40,000 volts BUT, its very easy to start loosing this high voltage by the ten-thousands in the delivery between the module and the spark plug. Bad caps or worn rotors make the transmission of the spark late, or slow Dirty plug wire boot contacts in caps will only let 1/2 the voltage get to the actual spark plug . Start out with a 36,000v jolt and it ends up being 3000v of low amperage by the time it gets to the spark plug tip. RPM increase demanding a faster turn around time for the HEI means it generates less voltage at higher speeds, so any other things that might decrease the smooth clean path this voltage takes will further lower the power at the plug tip. Compression makes it harder for the arc to take place. Combine a poorly atomized fuel/air mix, and there is almost no reason for combustion to take place.

When you trade AWAY the power of high amperage, for HIGH VOLTAGE, you sacrifice the ability to push past high resistance (dirty connections, large gaps etc) and that is exactly what HEI does. It takes 12v and turns it into 30 or 40,000 by turning the high amps of battery power and making a trade....amps to volts. As in ANY trade, someone gets the better part of the deal....

So, new cap & rotor, brass, NOT the house brand aluminum post. Get a good clean secondary ignition. HEI is a good system but it does not take much to make it weak and problematic. The corrosion and dirt under the dist cap
(coil cap)where the blade connections are, is a great place to start cleaning with a wire brush. The ground bar/wire on the coil is next...

-------------------------------- ------------------------------------------ ---------------------------

New fuel injectors. The most popular are the Bosch-III from FIC. These are the contemporary equivalent to the stock 1988 (older actually) design that's in your car now. Large UPGRADE.

The ohms test only tells you that the injectors coil is OK and able to cycle the inj spray. That has nothing to do with the inj ability to atomize the liquid into a vapor.

Its well known that the injectors from the 80's were NOT alcohol blended fuel compliant. They were not designed for alcohols and the seals inside get hard and brittle and cause the injector to dribble fuel out instead of an atomized spray. WET fuel does NOT burn and ignite under pressure (compression). Atomized, vaporized fuel DOES.

How can you test for this ? who knows...?
But I can tell you without ANY shadow of a doubt that there are FEW DD corvettes from the 80s with any miles on them that are running well with the stock injectors still in them. Some guys will never know because they never put their foot into the pedal....so the symptoms never appear. Until the day the car just won't start anymore, or it suddenly starts to buck and chug up what WAS an easy climb up a hill....

Ask any of the regulars here, the injectors are the number 1 cause of the problems you describe when there are no codes or other obvious signs of damage.

I hope this is of some help, take it or leave it. There are few other things that CAN cause your symptoms, and the age, miles and history SCREAM fuel injector.

Good luck !

 

Ok, so its not the maf. The output voltage did look ok but I kept hearing that if it runs good with it disconnected, its the problem.

A few years ago I checked the fuel leakdown and it held pressure for several hours. So it is much quicker than before. You mentioned leaking back through the pump. I guess it could also be leaking through an injector.

I'll put the fuel pressure gauge back on, tape it to the windshield and see if it holds under load. I'll be driving with the MAF disconnected.

The current (old) cap / rotor set is with brass terminals. I think it's Accel.

The only reason I'm working on the car is to try and get it through emissions (idle and 2000 rpm tail pipe sniffer - no dyno). I'm going to try it as is on Friday. If it passes, the car will probably get parked again until the spring. If it fails I'll pull the plenum and look for injectors.

Thanks!

Rob

 

When you do your drive, watch the behavior of the needle on the gauge....carefully.

What you want to see is a steady needle while cruising. It should drop as you accelerate...BUT, it should recover instantly as rpm become stable. The needle should NEVER flicker, wiggle or bounce. That indicates that the pump is unable to supply the demand and/or there may be obstruction between pump and injector. The gauge shows you what the FUEL is doing. Its actually showing what the un-used fuel is doing. the fuel that's being sent thru injectors is not detectable, its gone. You see the fuel that didn't go thru the injectors and what its doing now,. The needle is a reflection of the pressure or "tension" on the fuel. As fuel is removed from the rail, it gets replaced by the pump.
When the proper tension is present, there should not be a noticeable amount of time lapse while at ANY steady speed. This is also where the regulator come into the picture. It sets the amount of pressure that will be seen in the rails by dumping off any excess. The pumps ability to meet the minimum demand is what we must have. Whenever there is a sudden increase in fuel demand and/or rpm, the pump WILL be a split second behind and as rpm go UP the pumps ability to both maintain the VOLUME being used and the PRESSURE that is required, becomes the question. A bad regulator CAN make it look like the pump is at fault. There are test in the FSM that help you rule out the regulator as the problem.

If the needle on the gauge flickers ANY at idle, look at the fuel filter before buying a pump. Flickering is a lapse in tension....the inability to maintain a steady pressure on the system.

Expect to see the needle drop a LOT when you punch it, but it should recover right behind the rpm as they go up. If it does not, the pump is weak and should go.

Next storage, a bottle of SeaFoam will go a long way toward keeping the tools in the box when its driven next time. A FSM set will also be a good idea if you have to swap injectors and there are LOTS of good YouTube videos on this subject.

Lots of tricks and tips when/if you have to do injectors. Not a horrible job, but 1st time will be an all day deal. 2nd time is about 4-6 hrs. The 1st time you battle/scrape the glued on gaskets from the factory. Gotta be nice to the aluminum sealing surfaces too...

Hope this helps...

 

When I checked the idle pressure a few weeks ago it was rock steady. I'll keep an eye on the drop / recovery when accelerating.

I was planning on some StaBil in the gas for the next period of non-use.

I had the intake / plenum off back when the car was only a few years old. It was leaking oil through some of the intake manifold bolts. That was about 20 years ago so my second time will probably take as long as the first!

Thanks!

Rob

 

Had a warm day today so I decided to pull the plenum / injectors. I connected them to my test rig and two injectors looked good, four marginal, and two had major flow issues.

I thought they were going to be multec's but I guess those started in '89?

The EGR valve doesn't hold vacuum so it will also get changed. Also going to change the short "U" shaped coolant line. Any other "you might as well's" that I should do while its apart? I did spend some time cleaning things up. One valve cover was leaking so I pulled it and will replace the gasket.

Whats the general consensus on gaskets (valve cover, runners, throttlebody) GM or aftermarket?

Do any of the lower runner bolts go into an oil passage or galley? The front lower passenger side and rear lower driver side runner to manifold gaskets / bolts looked like they had fresh oil on them (did not look like it was seeping from the plenum or pcv).

Thanks!

Rob

 

...

 

Ah crapola. I looked at the valve and it does have the "N" stamped on it. Guess I read that section of the FSM too quickly. The flow chart talks about the SOLENOID holding vacuum. The EGR diaphragm should hold 10" for at least 20 sec. I'll use another car to supply the vacuum to test the valve and hopefully return the new one.

No TB coolant bypass for me. For a short time I ran a Holley 4Di Projection (was a beta tester) on the supercharged Rx7. In cold temps you could see ice form on the throttle blades. Granted that was throttle body injection (injectors above the butterflys) but I'd rather leave it for reliability than the minimal hp bump. But then again, one less hose is one less likely to fail!

I ordered Felpro gaskets. The factory gaskets I installed when the plenum came off in ~1990 were in great shape and came off the aluminum cleanly without any difficulty. Hopefully the felpro's have the little plastic studs built into the gaskets like OEM.

In looking at the runner bolts, there isn't any way they're going into an oil passage. Hopefully the oily gaskets were from the leaky valve covers and not from the intake manifold bolts. Leaky manifold bolts were the reason I had to take it apart back in 1990. I'm not touching them this time. Put the new gasket on the drivers side valve cover. Just tightened the bolts on the passengers side because it looks too involved to remove!

Thanks again!

Rob

 

ok, connected EGR to a manifold vacuum port on another car. Gauge indicated a steady 17" of vacuum and I could get it to spike above 20 when coming down from a rev. EGR valve diaphragm never even moved.

When the new one shows up I'll try the same thing. Supposedly the new one is also for negative pressure.

 

List price on that MAF is $1447.00. And they're selling it for $106.30. That's a great deal. Buy a few and re-sell them on craigslist for a profit.

 

Sounds a little too good to be true, at least to me. Thankfully I don't have an MAF.

 

You can use propane to see if there are any air leaks. Look at the scanner values. If your IAC steps are at 0, very good chance of a leak somewhere.

As to the injectors, they are junk so you might as well do them and if you are there, the Fuel Pressure Regulator could be changed. While you are there, you might as well take the throttle body off, take the IAC valve off ad the IAC housing off. Clean the assembly with brake cleaner. How are the plugs, cap and rotor? Wires?

Once you have done all of these, take electrical take and remove a hose off the manifold and seal it up, see if that changes anything.

 

Yup, cleaned it all up when it off the motor. T.Body had typical carbon coating, now nice and clean.

Waiting on a low mileage set of FIC Bosch IIIs that I purchased from a forum member.

Plugs are new, cap / rotor are old but still serviceable (Accel brass setup). Wires should probably be replaced but no arcing and ohm out ok.

 

Did you remove the housing and clean it up? New gaskets I trust?

I'd get new cap and rotor if it is old. Ohm wire doesn't mean much. Every strand of wire can be broken but one and it will Ohm good. How did you check arcing?

 

Yup, all new gaskets.

Arc check in dark garage w/water mister.

All of the parts came UPS tonight. Looks like it goes back together on Sunday!!

 

ok, the good news:

It fires right up. After letting it warm up (185°) and checking for leaks I take it out for a drive. Almost everything feels good. Mid to WOT is very smooth, no hesitation, bucking, or backfiring. Hot idle is rock steady, smooth, and 800 rpm. No pinging either so the new EGR valve must be working.

The bad news:

Cold start results in ~1300 rpm idle which slowly came down as the motor warmed up. But as the idle came down it wanted to drop too much, then it would rev up and repeat. After a few min it steadied out at a normal idle. Maybe I screwed up the IAC motor or left some gunk in there?

Trying to drive it on the cold motor is almost as bad as before the injector swap. Major bucking. Maybe bad coolant temperature sensor? The resistance / temperature check looked good a few weeks ago.

Only issue after it comes up to temp is at a very light throttle it wants to buck. TPS Sensor? Again it was right on the money a few weeks ago.

 

Get a scanner and see what the ECM sees as far as IAC counts. After that, set TPS and you can monitor engine temperature

 

Guess I'll do some searching on the C4 Scan and Tune to see what scanners work (and are cheap). Hate spending more on a car (my brothers) that isn't driven, but I want to find / fix what's broken.

Would be nice if I could use the old AutoTap or EFI Live V2 I have for my C5.