I have an 87 L98 that has been rough running since I bought it 5 years ago. Nothing serious, never dies, always starts. I replaced the plugs and wires and filters and cleaned the TB and IAC and checked the timing and TB adjustments and it had no effect. I had Jon rebuild the FI's and it didn't make any difference. I have great Fuel Pressure. Have checked for vacuum leaks and can't find any. It has a rough idle and obvious miss above idle that is annoying. I recently took it on a 400 mile trip and got 25.5 mpg, no irrational engine problems. Spark plugs looked identical, not fouled after the trip. It acts like I have a vacuum leak somewhere or a timing issue. Just a rough idle and slight stumble above idle. I do not throw parts at problems. The question is: where do I begin to troubleshoot? Compression test? Ignition ? Analyser? Anyone had any similar bugs or problems? I don't know where to start.

 

Have you tested the TPS? Does it have good values throughout the range?

Are you sure that there aren't any air leaks frm the air filter back? If there is an air leak then the values that the MAF report would be off therefore create problems running.

How is the distributor? Does the cap look good? How does it look on the inside?

 

I would test:
-maniflold vacuum
-fuel pressure (FPR vac line off & plugged)
-engine compression
-scan tool readouts

 

I've been going through the same process over the last week. I've been working out of Dyno Don's shop and everything has checked out. I did rebuild the distibuter also besides replacing wires and spark plugs. My next step is pulling the valve covers Monday and checking for broken lifter springs. A spring broken toward the top will give an intermitant miss that's more pronounced at idle, but will or might lessen or go away at higher rpms like mine does. The scan tool does help make the process go faster. I'll let you know what I find. Good luck.

 

I checked the TPS several times, used propane to try to find a leak. Replaced all the gaskets when I redid the FI's. It's interesting that you mention a broken lifter spring. I was considering going in and rechecking the valves. I had to replace the valve seals a couple of years ago and had it all apart then. Let me know how that goes. In the mean time I think I'll get a compression tester and get some readings.

 

check wires for insulation thats worn off from rubbing or vibration, the ones that are on the back of the distributor.

 

are u sure the firing order is right?I know its a stupid question but I had a chevy truck once and after changing plugs and wires for some reason
I thought check the firing order sure enough two plug wires were wrong.
when I changed the wires I did them one at a time so that meant the previous owner was the one to cross them!Truck ran and started pretty good too.Ran much better after getting firing order right.

 

Your EGR valve could be stuck open.

 

In the life of my L98 I have some rough engine problems,

1.- almost all was sparkplug wires, the solution for me was the looms that attached to valve covers.

2.- TPS- when mine fail, dont feel rough but certainly dont have power

3.- egr- Feels rough !, buy new plenum gaskets, if you dont have it yet, install adjustable fuel pressure regulator

4.- MINE IS MAP- this feels rough, more than other, 1 time the air vaccum hose coonector sucks air, other time the map connector was loose.
I still have the original 20 years old map, is good but I already bought a replacement.

5. fuel injectors, rochester injectors gave me several problems, bosch injectors are good option


IN your maf system , check maf and air leaks

 

Oh oh. I have oil in my radiator and overflow. No antifreeze in the oil that I can detect. Looks like head gaskets. I'll verify with compression test. Crud...I just got it put back together after I got the injectors cleaned. Thanks for all the suggestions but this looks like the source of my rough running. I'll post after I get the gaskets checked out.

 

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Well, the mystery continues. Compression test was 175 PSI in all cylinders. I pulled all the wires and cleaned and rerouted them. I checked the cap and rotor and pulled out the HEI and checked all the contacts. It seems to be running a little better but there is still the telltale stumble and rough idle. I don't know what the black stuff is in the overflow tank, it's a thin coating on the inside of the tank. I couldn't detect anything in the radiator after warming it up to do the compression check. I am not losing radiator fluid and oil and plugs look clean. I think I am back to chasing a ghost vacuum leak.

 

Usually my suggestions are completely wrong, but FWIW, here goes. If you were to have a bad head gasket, you could still be taking oil into the coolant without a detectable compression issue. A cylinder leakdown test may be a more accurate method to find a possible leak. Obviously I don't know how much oil you've found in the af, but it sounds like you have some sort of leakage in that direction.

Also, I don't know how many miles are on the car, but is there a chance the EGR passasges in the manifold are getting plugged up? And in that thought, maybe the idle passages in the Tb, plenum, or base intake? Just some thoughts, hope it helps.

I took a minute to re-read some of the other suggestions, and a long shot might be to check the endplay of the distributor. opinions vary, but it's been my practice to keep the endplay to .015"-.030". If the endplay is too great, the dist shaft begins to move up and down and the helical cut of the cam/dizzy gears cause the timing to become irratic and can cause drivability issues. Especially when you have a computer in there trying to compensate for changes while they're happening, but the mechanics of the dizzy keep changing and causing odd timing characteristics when the engine is under actual load. Also, if you take me seriously enough to pull the dist again, check the sideplay of the dist shaft for wear in the bearings. This WILL cause timing problems. If you're not too sure how much wear is too much, take it to a parts store and compare it to some new ones to decide if it's in good enough shape.

Again, I hope this helps.

 

Have you checked your throttle body shaft/bushings? I recently purchased an 87 and it too was having idle problems. When I checked the throttle body had all kinds of slop and was very worn out. I bought a BBK 52mm on here from C409, after installing it this week it now idles perfectly.

 

That was going to be my suggestion, too. What's the mileage on the car?