Ill try to make this as brief as possible, but the problem gets a little bit long winded.

Car: 89 L98 Coupe.
Mods: all stock
Problem: When the car goes into closed loop, the idle starts to swing. At first it would swing from 500 to 1000 at idle. Car drives fine at WOT and cruise save for low speed stops when it wants to pull through the brakes. No codes.

Fix #1
New TPS and IAC. Adjust TPS. Clean MAF. Clean TB and IAC bore. Clean fuel injectors. Visually check all listed grounds on car (except for grounds behind dash). All look good.
Result:
Idle only swings from 6 to 800 (ish) now. But NOW when you go to accelerate it bogs down and will not go WOT at all. Seems to have a starting and charging problem with some noise coming from front of accy drive system.

Fix #2
New Cats. Old ones were toast. Got new magnaflow cats. Replace. Also replace O2 at same time. New fuel filter. Replace old maf relays with new MAF relays per GM TSB. Seems to have charging issue from gauges on dash. Replace ALT (tested bad) battery (old and tested bad and even smoked when we tried to charge it) along with drive belt (worn) and tensioner (making noise and not holding proper tension on belt)
Result:
Does not bog so much, but still does. When you go WOT, the car will not go anywhere. Then the problems get worse. Idle swing there, but only from low 600 to high 700 range, but still there. Charging issue gone and no noise from tensioner.

Fix #3
New distributor. Took old module off to have tested and found too much play in dist shaft. Took it out and the drive gear teeth were worn pretty good. So new module came with new dist. Stab dist and retime engine at 6*. Starts GREAT now (didnt know it was a problem before). Security light kept coming and causing intermittent no-start problem on so replace ECM. New ESC module just in case.
Result:
Car starts good, but still hits a wall at WOT. No intermittent no-start problem anymore. Still no codes. Idle swing still there and problems persist and get worse after WOT run.

Fix #4
Test EGR on a hunch. Tests bad. Replace EGR pot along with new upper intake gaskets.
Result:
Car idles a tad smoother, but still has idle swing problem and growing acceleration problem.

At this moment I do not have a way to get any other data other than codes from the computer as I lost my OBD I cable for my scan tool. The problem is still there, not as prominent but in the mean time the acceleration issue has popped up. This car was all original with 40k miles.

I hate to keep throwing parts at it, but at this point, all the parts have helped to partially rectify the main problem as well as address other smaller problems that became more prominent. The car only really seems to have a larger problem with the hunting idle when in closed loop, but now has the acceleration issue at any coolant temp.

I plan to hook my fuel pressure gauge up to it again and take it for a drive while someone else watches the gauge. Any other places to look?

 

40,000 is about the time a weeping head gasket on the '89 becomes more prominent (alot started weeping close to the end of the warranty), so I'd take a look at that - #7 to be exact. But, an unstable idle is usually because the ECM is getting some signal that it needs more fuel, so I'd also suspect that air is getting pass the O2 that isn't coming through the MAF. Block off the Air Hoses to the Headers and see if that does anything. Aim some propane at the manifold and around the Intake to see if it speeds up. Check vacuum lines for leaks; particularly at the EGR Solenoid where it splits with one half to the EGR Solenoid and the other the Vapor Canister. Speaking of which, if the Vapor Canister is venting at idle, that will make it speed up, so clamp off the hose to the throttle body and see what that does. The only other things I can think of would be loss of the a/c signal. a funky Coolant Temp Signal, or a Park/Neutral switch out of adjustment, but you'll need your scanner to check those out.

 

I used the old school method of looking for vacuum leaks with a water sprayer. Usually that will make the RPM go down. No avail, so then I used something a little more combustible like carb cleaner on a straw. I used a whole can and doused every vacuum connection on the car. Still nothing. I will cap off all the vacuum connections and see what happens from there. That should help me see if anything is amiss.

It was suggested that the fuel injectors could be bad. I can see that as causing both issues, just wonder why one crept up after the other. The hunting idle and the acceleration problem.

 

it's a 85' coupe,probably the same problem,it's mine...

http://youtu.be/kG2W1FY4AA8

 

Check fuel pressure first, all your issues point to to low a fuel pressure.

-Lean idle means hunting idle.
-Bog at WOT could mean lean.

Take care of the basics first.

-Check timing
-Check fuel pressure
-Look for Codes and fix any

Verify all above is correct and then trouble shoot and replace parts.

 

Do the 89's have that crappy AIR system to? That's why mine ran so rich and hunted for idle constantly. May be something to look into. If you can hook a scanner up and see what its doing AFR wise once it drops to open to closed?

 

It does have the AIR but it injects directly into the exhaust. The AIR now shoots into an open cat.

I did check fuel pressure at idle, it was steady at 39 key on, then engine running it would fluctuate about +/- 3 psi.

I have not checked for driving pressure yet, but I will.

 

Don't exactly know how you got around the switching valve, but hopefully the check valves are removed from the Headers (or still working) because when they leak, it'll suck air pass the O2 which then fattens up the mixture. Leaking headers, particularly aftermarket, can do the same thing.

Fuel Pressure should have nothing to do with it (hunting idle) if it's steady and your test shows that it is.

 

Oh, you know what, I had totally forgotten about those. The diverter valve I guess it is called. HMM. I will have to check into that.

The car is stock so they have those black valves on the tubes that run to the AIR system. No headers, only factory manifolds.

 

The hoses don't matter, though if they're not connected to the Headers, they will blow air wherever they're pointed at startup (and of course be noisy). Checkvalves are simply one way devices that allow air to only flow in the direction of the header - or CAT, because there's one on that hose/tube too. Remove to make sure they're ok or you shouldn't feel any air out of the hose side with it running.

 

I did the diagnostic check on the AIR system per alldata. They appear to be working how they are supposed to, although it does feel like a small amount of air is getting by. I did not have time to take the valves off to see if they were actually blowing and cap off the exhaust lines. Honestly around here, that AIR pump/valve assy should be done away with. I am sure it could get rid of a few problems.

On a more interesting note, my co worker and I took the car on a test drive with me holding the fuel pressure gauge. At idle it was 37 as noted. Then we went down the parking lot. It would actually lower to 30. Then during cruise it would fluctuate from 30 to 39 on wot it ONCE went up to 43 but then dipped down and fluctuated between 25 and 35. We brought it back and did some hard accel runs in parking lot, and it would then almost die, back fire, studder, and the pressure would drop to 15. Any type of accel from then on was under 29 psi with it dipping to 15. With the car at an idle then, it would slowly climb to 30 but stay there with it running rough and still hunting.

I went to take the gauge off and just let the fuel pressure bleed off. It was then I realized that the fuel coming out was not clear. It was tainted. Rusty in color. I let alot of it drain, and sure enough, rust all over in the gas. I let the remainder drip onto the concrete and it rust stained the concrete.

So now I know what that means and why the fuel pressure was acting odd and why it is progressively getting worse.

 

Whats that mean where there is rust in the fuel like that?

 

It means the gas tank and/or fuel pump is rusted out. My guess is this car has been sitting for so long with either bad gas, gas with a large amount of water in it, or ethanol based fuel, or todays gasoline (with 10% ethanol or more in some cases), that it caused the tank/pump to rust out. And this rust is causing all sorts of problems.

Really the only fix I see is to take the tank and pump out and get whole new assemblies. Then take the fuel filter off replace with new. Check and clean the hard lines, replace if necessary. Change FPR. New injectors. Clean rails.

Anyone care to comment?