Wondering if I could get some help with a problem I have with my 85.

It is backfiring thru the intake if you punch it but doesn't do it if you get into it slowly.
This started out as what felt like a miss in the engine. I have replaced the plugs,
plug wires, distributor cap and rotor button, idle sensor, and cleaned the maf
sensor with the cleaner for that designed for that purpose.
I also ran a fuel injector cleaner in it but no change.

I have looked for vacuum leaks and cant find or hear anything.
It acts just like a timing issue but I have brought the #1 to tdc, pulled the distributor
cap and the button is on the #1 post. I have checked the timing and set it by the
book to 5 degrees just to be on the safe side.

The engine only has 35 thousand miles on it since it was rebuilt and a month
ago it ran like a top.
Any ideas would be greatly appreciated.

 

Backfiring thru the intake is usually the sign of a lean condition. Did you check the fuel pressure?

 

No but I will round up a guage and check it. I just viewed a video with an 88 that had the exact same problems and cured it by replacing the fuel
pressure diaphram.
Thanks for the response

 

I would pull the plugs and see if one looks different from the rest. If it does a compression check is in order.

 

Finally got a gauge. When I just prime it, I get a reading of 32 which quickly drops to 15 and then takes about 5 minutes to go to zero.
I have seen some videos that said that indicates the pressure regulator is leaking.? I pulled to vac hose off the regulator and it does
smell like gas.
Your thoughts
Richard

 

I would check rocker arm adjustment. Also verify all cam lobs are still present.

 

fix the pressure reg.

 

I was going to suggest that. A similar thing happened to me a mechanic said it's in the valves. I pulled the valve covers and hand rotated the engine. The intake lobe on the number three cylinder was gone. When I pulled the cam it looked like a journal. Perfectly round. That is why I suggested the compression test.

 

I think that's what I am going to do. I have had cams to go bad before and
typically it is something that happens slowly . This happened in the matter of
a week or so .
I will let you know how it turns out. Thanks for the input from everyone
Richard

 

Funny thing about my 1984. I was running great. Got on the Garden State Parkway, NJ. Doing about 80/90. When I got off the GSP I had a backfire. the car had 77 K miles on it. But, I'm sure they were rough miles. The front clip was from an 88. Put to my point from running really good to what is that in a matter of hours. Good luck

 

Just be sure to order a new Fuel Pump Regulator Diaphragm designed to operate with the 'New" oxygenated gasoline we have to live with now. Do Not Buy one on Fleabay, I have seen junk out there I would not use on mine. Don't buy "new old stock" diaphragms as there have been some out there. When you buy it get it from somebody who sells enough to assure you of a fresh new regulator diaphragms. Zip Products, Ecklers or that level supply house is best as they sell zillions of them. My favorite place is NAPA central warehouse as they have a constant fresh supply as well.

I have never tried the aftermarket Adjustable fuel pump diaphragms as I have a stock engine. If you plan to have any major modifications to this engine you might consider it now while you are putting a new one in there.

How is the EGR system working properly? The EGR is an important part of the L98, it keeps the emissions lower and makes more power with the EGR working. If the EGR goes south like mine did they are a PIA to fix and clean up from. I would not have noticed but when they did the emissions test they could tell right away by the Higher NOx numbers. When it is stuck the car can actually run hotter on trips and might even start to ping or detonate. A non-functioning EGR is hurting your engine.
On the left front of the engine there is a device connected to the Thermostat Housing hardware, it is a little EGR Solenoid and uses Vacuum to operate the EGR valve. Be sure that the solenoid has the vacuum lines attached and the wiring attached as well. The signal from the ECM is going to be Pulse Width Modulation which makes it harder to check out with most multi-meters. The last test on the EGR valve is to disconnect the hose coming from the vacuum solenoid and attach a hand held Vacuum pump, pump it up and watch the EGR diaphragm for movement. It should be able to go up and down and change the idle while doing so.

My old 1988 C4 with the 700R4 transmission was able to break into the low 30 mpg range on a flat interstate with cruise control and the AC blowing cold. That is not why I bought it but it sure makes operating it more fun!