PCV system on 89 5.0 engine? Improper config may have cost us the motor.


Active Member
Jan 9, 2022
Southern California
After all the troubleshooting with my Dad's 65 Mustang, and the weird issue at the last post, it turns out the pistons had a bunch of carbon buildup on the top and the rings got stuck to the piston grooves and both the pistons cylinder bores got all scored up. All the cylinders on one side are at about 90psi except #1 which had 0. The heads are OK. The car is getting a new short block. The theory we have is that the PCV system is not in place or failed. I know we have a hose going from the filler on one valve cover straight to the throttle body. I read this post, https://stangnet.com/mustang-forums/threads/new-5-0-build-pcv-breather-both-or.925401/ and it sounds like that is only part of the setup. The PCV also has a valve at the back of the intake? Point me in the right direction, guys. This new engine and labor is set my Dad back $6000. I do not what this to happen again. The car is still in the shop, so I can't look at it easily but I can go there to look.
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Look at the inside of the intake. Is it clean or is there oil/other junk inside the plenum? Look through the throat where the TB bolts on with a light. If there is oil or other junk inside, if there is visible oil a inspection of the PCV system is in order.
Without any info, like a modification/build thread, I do get a sense some familiarity, but I've been confused before.
Some lower intakes have a smaller baffle riveted to the underside for the PCV, some discussion about engines sucking oil with the smaller baffle.
General karthief,

This is the build in my Dad's 65 Mustang that I had the long thread about and then a shorter thread about fuel pressure where the new fuel filter may have been the cause.

To answer your question , I have had the upper plenum off but I don't recall seeing anything like oil in it. I didn't look into the intake after removing the upper part. I just covered the holes with painters tape it to keep anything from falling in.

Since the top end came off a donor car a good 15 years ago I don't have detailed knowledge of what they put on this car. I know some items were not transfered like the purge canister and I think the EGR valve. I wasn't involved.
Does anyone have a reliable web site that will explain the common causes of carbon build up? I see a number that sort of cover the topic, but often end up talking about direct injection issues. The most common is conditions that lead to unburned fuel. With the EEC I'm not sure how to tell. The O2 sensors and MAS should be able to adjust for that, I would think. Error codes should be thrown when these devices have issues. At least that is my thought.

Now for another question. If you read my last post in the linked thread, I was just about to button up the car, and take it for a drive, but, when I tried to fire it up to drive off the lift the car ran like crap. Surging, backfiring out the intake and stopping. I made sure all the connections I may have bumped into, removing the hard fuel line I had temporarily in place to watch the fuel pressure were tight. I made sure the distributor didn't move.

Could these backfires harmed the throttle body and electronics or the MAS sensor? Would it be best to send the computer back in for repair to get tested?

Does charging the battery with the cables connected harm the EEC ? My brother had read that was a bad thing to do.
Carbon build up can be from blow by i.e. oil in the cylinder and then getting cooked onto the top of the piston, excessive fueling either by incorrect fueling or misfires.

Too much fuel will just wash the cylinder out. The fuel will remove the oil from the cylinder walls which means no lubrication for the rings which prematurely wears them out. This will also cause a new motor to completely oak to break the rings in and result in low to no compression.

If the MAF or O2 sensors were out of range the ECU would throw a CEL.

The ECU is a minimum of 30 years old so the caps are at their end life so sending it to ECU Exchange is t a bad idea but it may not fox the issues. Do you have a known good one you can swap in and try?

A back fire could have eased with the MAF sensor but one would think the EXU would throw a code if that happened. Have you cleaned the MAF sensor? If not that could help. They sell MAF sensor cleaner at the pad stores. I spray it down then GENTLY use a q tip that has the cleaner on it to kind of brush the sensor filament. It’s delicate so be nice to it. Let dry and reinstall.

If your battery charger is a super high amperage charger then it might mess with it but our standard 20A charger is fine.
"The ECU is a minimum of 30 years old so the caps are at their end life so sending it to ECU Exchange is t a bad idea"

I sent the ECU to them a few months back when I was in the middle of the thread I posted above.

"Have you cleaned the MAF sensor?" I did, back in the same thread, but then replaced it after that.

The battery charger has a number of settings from 4 amps up to 200amp starting mode. One of the rectangular box with a handle and wheels.

" If the MAF or O2 sensors were out of range the ECU would throw a CEL."

I posted the codes I was getting before and after changes in the above thread as well. I changed out the MAF sensor trying to resolve one code.

I guess what I am trying to figure out is whether the car has been setup in a way that was not burning off all the fuel all along and I happened to come on the scene late in the day or not. One thing I read was that a car not up to full temp would also add to the issue. Dad's car runs cold. It's a perfect Parade car. You can crawl along just over idle and it will not over heat,. He was in the Hollywood Christmas Parade with some Disney " Stars" sitting in the back seat. No issues at all. It takes a while to get the thermostat to open, running in the garage. He feels the aluminum heads help that.

The car has probably not been driven on the street for at least a year and a half. I've been testing it and replacing parts and just bringing it up to temp to read codes and test fixes about that long. Could just that, be a cause?
The ECU gets input from the ECT sensor (engine coolant temperature sensor in the coolant crossover tube between the water pump/intake and heater core) and uses that information to determine when to exit open loop (where it ignores the O2 sensors) and go to into closed loop (where it uses the inputs from the O2 sensors). The ECU also gets input from the ACT sensor (air charge temperature sensor in the #5 cylinder runner on the lower intake) and uses that data for fuel input as well. The ECU uses both of these sensors to determine fuel input so make sure they are in good shape.

The ECU goes into closed loop based on time and input from the ECT sensor. With that the quicker the engine hits that temperature the quicker it enters closed loop. The temperature the thermostat opens at will not necessarily be the temperature of the engine as that is just the temperature it opens to allow coolant flow into the radiator.

If your car is running cold then my guess is there is a pretty low temp thermostat in the engine and you have a really good cooling system. I would not run anything lower than a 180 degree thermostat to aid the ECU in getting into closed loop so it utilizes the O2 sensors to burn less fuel or at least attempt to.

My car has aluminum heads, upper and lower intakes, phenolic spacer between the intakes, aluminum radiator, Mark VIII fan, and a DC Controls fan controller and I can hit operating temperature pretty quick in the garage. I run a 180 degree t-stat and the Autometer temp gauge will show right around 190 degrees at a light or in light traffic. On the highway it reads right around 180 or a little higher.
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Thank, Aero. I like a good explanation.

I visited the car at the repair shop and I checked out the parts and talked for a good while with the mechanic/owner. The pistons and heads and valves were really not that bad. The passenger side pistons and cylinders were scored. The rings on them were stuck. The timing chain was rather loose, which was odd, given the car is a cruise and show car, and not driven hard. He felt the head gaskets were not what he would consider quality, the heads just fell off the block when the bolts came out. We did one when I was there. While the engine showed some signs of running rich, it wasn't excessive. In his view, my Dad did not get the quality " new in crate " Ford engine he bargained for and the wear was done over a long period of him driving it. Call it a not very quality build.

I checked what i believe was the PCV valve at the back of the intake. There was some oil in it, but nothing really in the lower or upper intake. Since I ordered it I'm going to replace it. I may do it at his place while we're waiting for the new engine to arrive. It would be a lot easier to get to!

I am sending the ECU back the the Exchange place you guys pointed me to, just to get it checked out. Can these computers be programmed at performance shops, if needed or are they just setup at the factory to match specific engines? The shop suggested I have that done, to be sure, if it can be programmed.
I would strongly suggest you look into a custom tune that is done on a dyno. You will get the most bang for your dollar. I run an SCT chip on the A9L EXU in my Coupe and it’s worked very well over the years. I would think there would be a few shops in SoCal that still tune on the EEC-IV dinosaurs. When I got mine many years ago it was $500 for the chip and tune.

Another option would be to go the MSPnP route or similar but that can get expensive.
We have been talking to the repair shop guy, who we've known for about 9 years. He has told us the price for 302/5/0 motors has really gone up in the last handful of years. Often you will see an advertised price, but it's not in stock or the real out-the-door price is higher. Even rebuild parts are out of stock.

Does this appear to be the case for anyone whos been looking for a motor? I've sent to email to a handful of companies within 100 miles to see what they say. The motor that was in the car was supposed to be a new crate motor with roller cam and it didn't last.
Update and follow up questions.

The motor is out and we're taking it to a machine shop for a short block rebuild. Here is what I can see with the heads off.

1) The combustion chamber is black. The valves have carbon or real deep soot on the them. I know the tail pipes have soot on them as well.
2) The #1 piston was pulled out and the rings are stuck in place in the grooves on the piston. There is no real scoring on the bores as I feel them with my finger. This piston had 0 psi compression. The other three on this side had 100 and 90 and 100 psi. I didn't check the other side after finding the 0 cylinder. No damage to the piston I can see.
3) The shop that pulled the engine for us didn't like the head gaskets much because the head gaskets had no adhesion to the block or the aluminum heads. They would have fallen off the block when the bolts were removed if you didn't have a hand holding them. That seems odd to me, but may not be a culprit.

A theory is that the car was running too rich and maybe washing off the pistons. I would think that the car would have running very badly if that were the case.

A summary of what I found in troubleshooting in the past.

I found the fuel pressure regulator was bad and fuel would come out of it with the vacuum line removed. I replaced it.
I think I had some codes that told me the MAF sensor was out of the proper voltage range, so I swapped that out too. ( I'll pull up the older long thread to get that code. )
I sent the ECU out and they repaired it, since they found things that need repair.
I never got a code that told me that one bank or the other was running rich or lean.
I pulled all the injectors and had them rebuilt at a local shop. ( NAPA in this case. )
I tested the fuel pressure and had about 32 at idle and at full throttle would go to about 39. This was after the regulator was installed.
I sent the ECU back out to the Exchange to make sure I had not damaged it in some way, and they found nothing wrong and are sending it back.
The car does not have an EGR valve or even the spot it would bolt onto or the purge canister. I don't know why. ( Is this important ? )

I've been trying to Google other reason the engine would be running so rich and I found two things in a thread in another Mustang forum.

" The harness for the O2s is different for an automatic or manual car "

I don't know what this harness and top end came off of 12-14 years ago when my Dad and friends installed it all. The ECU is an A9L model, so I would hope the rest matches the car it came from. I think I recall looking that up and found it was for an auto trans.

Is there anything I need to check for that I have missed ?

Is the symptom all just related to fuel mix or is the engine, which was supposed to me new from Ford 5.0 just not built well?

I don't want this to happen again, of course.