Any advice would be greatly appreciated. I recently purchased a '94 Z-28 LT1 with LT4 modifications. Here is a rundown of the modifications on the car: LT4 Heads, LT4 cam, Edelbrock intake, Headers, Flowmaster exhaust with dump, 30 lb fuel injectors, high flow fuel pump, NO catalytic converter, cold air induction tube, MSD blaster coil.

The car is running rough. It hesitates at all rpms, spits alot of black carbon from the exhaust, and backfires with a black cloud sometimes when punched. The car has been driven less than 2,000 miles in 16 months, so I thought maybe the fuel injectors were dirty and the car just needed to be run. Also, the "Service Engine" light cycles on and off. The previous owner said he had the code checked and it showed a rich mixture. He believed the larger fuel injectors were throwing the code, but working properly. I no longer believe that. Google searches lead me to believe that the fuel mixture is indeed too rich. My first question is: if this is the problem, how do I fix it?

Another odd item is that the last time the oil was changed (over 1 year ago, but less than 3000 miles ago, according to the sticker), they used 20W-50 oil. I just changed the oil and used the manufacturer's recommended 5W-30 oil. My second question is: why would they use such a thick oil? To hide engine problems? I don't know if the car is burning oil, because I recently purchased and haven't run the car enough. I was told the car has only 30,000 miles since an engine rebuild. Chassis has 105,000.

My last concern is the current location of the oxygen sensors. Since they installed headers and removed the catalytic convertor, the oxygen sensors (currently 2) were moved from the exhaust manifold to the exhaust pipe, just after the headers join the exhaust. I'm wondering that since the temperature at the current location of the sensors is lower than the original location, is this causing the sensors to operate improperly? I hope that the rich mixture is the problem and the fix is not too complicated. My concern is that the problem is a serious internal engine issue.

The car will be a blast as soon as I solve the problem, but I hope I don't become disappointed and sell the car prematurely. Thank you in advance for your responses and have a Great Day!

 

First thing is it would have needed a retune for the modifications (especially the injectors). If the O2s are too far downstream, they won't stay hot enough to provide proper feedback (some guys with headers should chime in). Some real time scan data will tell you if the O2s are working and tell you any trouble codes. You will also be able to monitor other parameters to see how they look.

 

Is it the "LT4" cam (not a whole lot different than the stock LT1 cam), or the "LT4 HOT" cam (more agressive than the LT1 and the LT4)? Does it have 1.6X roller rockers?

Was it tuned for the modifications, including the injectors?

Normally, the O2 sensors are placed in the header collectors, and work fine on the 94-97 cars with 4-wire heated sensors. Only the 93's, with the 1-wire unheated sensors have problems. But EXACTLY how far past the header collector flange are the O2 sensors located? What headers are they? How were the wires extended - with professionally made harness extensions, by someone splicing the harness wires, or by someone splicing the sensor wires?

Scan it for codes..... don't rely on what the previous owner told you.

What was the oil pressure at 2,000 RPM with the 20W-50 oil? Did it drop significantly when you switched to 5W-30? If so, what is the oil pressure at 2,000 RPM with the 5W-30?

Running rich is seldom due to an "internal" engine problem, but it you continue to run excessively rich, you can cause significant internal damage as the excess fuel washes the lubrication off the cylinder walls, and dilutes the oil.

 

O2s need to be on the collector of the headers for the optimal reading.

As far as oil viscousity goes, here's my results, 111k on motor-

*Bought car from previous owner- 5w-30 hadn't been changed in awhile - 15psi hot idle/ 35psi hot WOT

Changed nasty oil and used valvoline 5W30- 20psi hot idle/ 40psi WOT
Another flush 500mi later same results.

Ran 1 cycle of yamalube 20W50- hot idle 25psi/60psi hot WOT

Switched over to yamalube 10W40- hot idle 20psi/ 50psi WOT, still what I'm currently running.

Also, these numbers are not from a digital reader, just basing it on the position of the analog oil pressure meter. This motor has no erratic noises whatsoever. I change my oil every 1000miles btw. If your climate is pretty cold (well below freezing) in the winter, I'd use the 5w30.

If you are worried about the thicker oil covering up a bad bearing- simply drain oil into a clean container and carefully pour it through a fine strainer to see if there is any bearing material. You can also cut open the oil filter to look for material there as well.

James

 

Your post is so hard to read with no paragraphs!

If you could break it down I will try to help.

Rich

 

I appreciate everyone's advice! I can now move forward in the right direction. I'll have it scanned and tuned professionally.

I am new to the message board and appologize if my formatting is not easy to read. I will improve that in the future.

After the oil change the oil pressure has not changed significantly. I already disposed of the old oil, so I cannot inspect.

I don't know alot about the modifications, except what I already posted, but your advice is exactly what I was looking for.

Thanks for all of your posts!

 

I can fix your original post.

 

Like shoebox said earlier, if the PCM was not tuned for the bigger injectors it is definatly going to run rich. If I were you the first thing I would do is buy a cable so you can scan/log your car. Then I would get a mail order tune from pcmforless.