A little background on my problem: Bought this nice white 1989 C4 coupe with a 6speed ZF last April and it was running like a top, started right up and idled very smoothly. I drove it all over the place for about 1,500 miles with no issues. One day, after fueling it with $20 worth of regular I pulled out of the gas station and it starts to run rough like it’s firing on just 6 or 7 cylinders.
I suspected bad gas and thought it would go away if I put in better gas but it persisted so I started to poke around under the hood. I noticed that two of the plug wires were shorting out against the exhaust manifold and one was cracked and quite brittle so I changed them for a new set of Belden Max 8mm Blues from NAPA.
Back to smooth running but just for a few days. So I poked around some more and decided that I did not know the story on the injectors and three of them looked a little different from the rest so I bought a set of Bosche 3s from FIC and put them in. They look real nice with their yellow color but no change in performance, still running rough. Next I changed the fuel filter. Still rough.
So I took the car over to Corvette Mike’s house. He is a near neighbor who lives in the adjacent town and was kind enough to put his skills and his scanner to work. We also tested for fuel pressure which came in right on the money. But I guess I flunked the leak down test. It went from 39lbs to about 20 in less than 2 minutes.
How bad is that, what are the repercussions and does it have anything to do with my rough running?
I have numbers from the scan if it would help.

 

I would go ahead and change the cap and rotor (distributor), spark plugs, air filter, pcv valve, and give it an oil change to finish up the tune-up. If that doesn't work, you might want to change the fuel pump and while you're in there inspect the fuel sending unit.
You could have fouled plugs because of the bad wires. Good Luck!

 

Thank you for your suggestions. When I looked at the plugs they all looked brand new and properly gapped. None are fouled. The oil was changed and so was the air filter. I will change the distributor cap and rotor and pcv valve.

 

Your leakdown test shows it's losing fuel and that's usually an injector which will fill up a cylinder (and you'll see which one by pulling the plugs), though it could be the Regulator, Pulsator or Pump Check Valve. Block off the Return Line and if holds it's the Regulator; if not it's an injector. You can then block off the Pressure Line and if it holds, it's the pump check valve allowing fuel to escape back into the tank. Don't drive it if it's an injector because a severely flooded cylinder can bend a rod or blowout a head gasket - see below:

Check #7 for a leaking head gasket - all too common on this Year and a lot started dribbling with very low miles - like under the original warranty. That's often the cause of the driveability issue you've described and many owners don't discover it until it starts missing; backfiring etc. Unfortunately, by that time it's eroded a gully in the deck of the block, so to fix it correctly you're either machining the deck or putting in a short block.

 

When I scanned Joes car, IIRC it was running a little rich. Everything else looked good. Im a hack when it comes to using my scanner BTW...
The timing looked good, it was advancing, TPS was at .49. After it dropped to 20 psi on the fuel rail, it held for a short bit until I took the gauge off.
Whats the opinion on the 02 senser?

Mike

Hey Joe, you can borrow my fuel pressure gauge to follow SunCr's suggestion, Or bring it back over to my house and I will help you.

 

You need to use a real leakdown tester. They aren't that expensive $50-$70 at Harbor freight and see what your motor actually does. Cylinders will lose pressure because of air leaking past the end gaps so that is normal. I'm not sure what is normal leakage/time.

 

There is no mention of a check valve in the Haynes manual. It states that the pump is a positive displacement type which I assume should not let fuel back through it by virtue of its design.

Where is this check valve?

 

The leakdown test he ran was on the fuel pressure. Sounds to me like the same results a lot of us have. Most likely bleeding back to the fuel tank through the pump.

I'm interested in your thoughts on the head gasket. Why would #7 be a particular problem? Wouldn't a head gasket problem should show itself with a compression test?

 

Thanks, so that's why it's important to read the original post.

 

Thank you Corvette Mike for your generous offer. I went to Kragen today to look at ramps and they have Rhino Ramps for $40 but they looked so flimsy and narrow. My front tires are almost twice as wide as the width of those things and they have a raised edge about half inch. I was about to buy them to install a new o2 sensor as you suggested might be my next move. Are those ramps safe? Does anyone think the o2 sensor might be the cause of the somewhat rough running?

 

The standard 8,000 lb. Rhino Ramps are more than sturdy enough to support your car. That's what I use on all three of my vehicles (including my wife's Odyssey minivan).

If you want to, you can buy the 12,000 lb. Rhino Ramp Extreme, but unless you've got like a full-sized SUV or pickup truck, there's no need.

And don't forget a dab of anti-seize compound for the new 02 sensor. Sometimes what they give you isn't enough to prevent the sensor threads from galling/corroding in the exhaust bung during its service life once its torqued in.

Stop by Harbor Freight Tools and get a crow's foot 02 sensor wrench. They run less than $10 and make the job a lot easier.

 

Before you spend any more money on replacement parts you need to diagnose why your FP is bleeding off. Perform the test procedures that SunCR suggested and let us know the results. You can access the fuel lines by removing the fuel fill door assembly and crimping them with vise grips. The line on the right is the pressure, the line on the left is the return. Make sure you pad the jaws some how(wrap them with duct tape) so you don't damage the fuel lines.

When you replaced the injectors did you also replace the FPR diaphram? If you pull the vacuum hose from the FPR is there fuel present? If so its leaking and needs replacement.

 

#7 is where allmost all experience head gasket failure (at least around here), but GM never explained why. Their reasoning was that leaking intake gaskets set up galvanic corrosion which ate away the head gasket. Their solution was to counterbore the front and rear intake mounting holes for intake gasket retainers, but only the '91 Vettes and ZZ4 long blocks with the current/replacement 113 aluminum heads have this feature (and the Vette intake gaskets with the retainers and the coolant restrictors are really hard to find). That maintains perfect intake gasket alignment which stops the leaks and the failure rate seems to be quite a bit less for the '91.

My theory is that the coolant restrictors on the rear of the intake gaskets allowed the crappy stuff to simply puddle on the rear of the heads and that's why #7 gets eaten up first. GM put those restrictors on the Vette (only - the F Body doesn't use them) to keep coolant at the rear of the heads longer to prevent hot spots; and in hindsight, it doesn't seem like it was such a hot idea. I found this site nearly 10 years ago because of the problem - many of us were discovering it with fairly low miles - mine was probably leaking by 25k. Poking around the net - like the defect section at www.nhtsa.gov, I turned up several lawsuits against GM about the issue - mostly from owners that found it shortly after the warranty lapsed. The complaints were quite old and have since been deleted. That seemingly led to the Service Bulletin which has been edited and now omits the phrase "to stop the high incidence of galvanic corrosion" along with some other language that makes the Bulletin appear quite benign.

As you may or may not know, GM no longer pumps coolant through the intake on all most everything they make. Some use composites too, though early examples, particularly on the 6's cracked and caused other problems. And some - aluminum and composite intakes - started sucking oil taking what GM called a "gasket set" from normal heating/cooling cycles. I haven't a clue as to why they can't or couldn't get it right; some experts blame it on the elimination of asbestos in modern gasket materials. The elimination of dissimilar metals - more aluminum blocks with aluminum heads has alleviated most if not all of the woes.

Compression testing doesn't always turn up the problem until it's obvious - new coolant that looks like crap within 6 months, plugged up heater cores, radiators, rust on the plug threads or an electrode that looks different from all the others, and/or an occassional Code 44 - super lean mix, are things that can clue you in.

As to your scan, if it's rich, it's more than likely dumping fuel from a leaking injector. Coolant, even in small amounts, leans it out - not that yours isn't; the leaking injector could be masking the problem. In any event, I wouldn't own another L98 ('88 through '90) without investigating this issue. A lot have been fixed, but they keep breaking because they weren't fixed correctly; ie, the deck is no longer square and they're still sucking coolant pass the new head gasket on the downstroke and pumping exhaust into the coolant on the up.

Check valve is part of the Pump Assembly and as far as I know, not servicable; ie, you gotta replace the whole pump.

There are flexible hoses just below the Tensioner which can be clamped off with some Vise Grips, but wrap a shop towel around the hoses.

 

You need to figure out where the fuel pressure leak is occurring. It could be at the pump, the fuel pressure regulator or the injectors. As the injectors are new, my guess is its probably at the fuel pump. Test as described above and find out where the loss is coming from. My guess is its from the pump, most likely from the pulsator as they are prone to leakage.

Hopefully its not a blown head gasket. Did you change the fuel filter with the rest of the tune-up? Check the vacuum line to the fuel pressure regulator for fuel.

When I had my fuel pressure problem I called Jon at FIC for his take on it. He walked me through some procedures and gave me specs and suggestions. He was instrumental in my diagnosis, along with the guys here on the forum. Being you got your Bosch III's from FIC, give him a call.

 

I first read about #7 causing problems back in the 60s. Because of it's position and internal airflow it runs leaner and stagger jetting was resorted to in order to address the problem back then.

Jake

 

Befor you spend a lot of money, go to Chevy, buy a new fuel pump plate gasket, about 1 foot of 5/16 fuel injection hose and 2 small hose clamps. Take out the pulsator from the fuel pump pressure line in the tank.
Once you have eliminated that flimsy troublemaker, then redo the fuel pressure leakdown test.
THe pulsator was a great idea, but a bad execution on GM's part.
Take that thing and throw it as far as you can.
The biggest problem with it is it is over 20 years old, and the silicone rubber has stretched over time, and there is no way to clamp it to the line to eliminate leaks.

The problem that I find with 'Ol #7 is it runs lean because of the intake configuration. GM added that little hot water tube on the L98 to keep water flowing in that area, eliminating hot spots. Don't know if it works or not, but i am not going to eliminate that hose , either.
as far as the car running badly,
you need to determine if it's electrical or fuel . IT could also be a vacuum leak as well, or a bad sensor (O2, temp, ignition control module, esc, etc.)or FPR, or anything. Just be persistent, and you will overcome.

 

SunCr,

Thank you for that detailed post. I knew about the head gasket issue because I went thru it my self. I had no clue to what was really going on. This forum is so awesome when we can all learn.

Thanks again SunCr. I usually always learn something from all your posts

Mike

 

Thanks, but I wasn't the first - I stumbled across this site looking for info and unfortunately, most of the guys who educated me are long gone. By the way, if you go without the restrictors; ie, use the F-Body intake gaskets, you don't need the bypass from the Core. That can be a plus since that molded hose is long out of production. Our Vettes were - and still are to a some extent, a beta platform for all the cool things GM wanted/wants to try. That one didn't catch on, was never used on the F-body L98 (or anything with a 350 in it) and GM finally ditched it trying reverse cooling with LT1 (next and that didn't last either).

 

Why not just replace the pulsator with another one vs. replacing it with fuel line?

 

Ok, so I bought the Rhino Ramps and they indeed do work just fine. I will be changing my own oil from now on.
I also changed the O2 sensor as well as cleaned the distributor cap and rotor with a little light sanding. While I had all those wires dangling I rerouted the spark plug wires and added a set of separators to them to keep them from touching places they should stay away from. I recommend to anyone having rough running or idling or generally not running as smooth as you would like to make sure your spark plug wires are properly positioned and separated with the use of retainers. If you do not have them, get them.
My C4 coupe is now running like a top and I am very happy. I plan to drive it like this for a few days to see if this effort holds up before I pester Corvette Mike for a retest of the leak down issue.
I also thank all the helpful people for their advice and I believe I will know what to try next if this problem comes back. But for now I'm back in the saddle again!