911 Engine Advice

Avant · 03:19 PM

I recently purchased a 2.7 engine with factory CIS system that is going into my project car. The engine is a stock engine internally and has the updated chain tensions. Assuming I was told the truth by the previous owner, the engine had about 45,000 miles on it, and he ran it with carburetors. The engine was sitting for about three years, and then another 8 months with me. Now, of course, I purchased this engine knowing full well that it might need work, and knowing that there is the possibility that I was told inaccurate information about the engine. That being said, I have gotten the engine running, and I have done the following to it.

1. Valve adjustement, and confirmed that head studs can hold torque.
2. New spark plugs, plug wires, cap and rotor, exhaust system is new
3. MSD ignition system with the MSD coil, per Pelican technical article
3. New fuel injectors, with the black round seals replaced. Still need to replace the seals in the manifold, but a spray test confirms they are sealing OK for the time being.
4. Fuel system is pretty much new, including a new fuel pump and filter. Injectors seem to be getting fuel, and spraying fine.
5. Checked for vacuum leaks, and corrected anything I could find.

With all that accomplished, my engine starts now, and runs but I have had a difficult time getting it dialed in. By that I mean I get backfiring through the intake, which I can't seem to clear up by adjusting the mixture. I did a compression test, with throttle wide open, and engine warmed up. I was surprised to find very low numbers. My numbers are between 71 and 85 psi. I thought maybe my cheap Harbor Freight compression gauge was off, so I tested with a borrowed gauge (same model gauge). I got essentially the same numbers, perhaps increased by 5 psi. Assuming my rings were shot, I then pumped in a small amount of oil into a couple of the cylinders. With this, I got a reading of about 110-115. Subsequent cranking and readings saw the compression go down a bit each time. What was strange to me, was that when my numbers went back down to their "dry" numbers, I tried to inject the oil again, thinking I would once again see the wet numbers in the 110-115 range. It was puzzling to me that I could not achieve the wet numbers again.

Is this a case of rings not sealing because of sitting for a long time? If so, can anything be done to get things back in order with regards to compression. Is it rebuild time? The engine does seem to have power, although throttle response is no good in the early rpms.

Thanks

Avant · 10-25-2015, 03:17 PM

I just checked my compression gauge by pressurizing it from my shop air compressor. It seems to be reading correctly. So that rules out a faulty gauge.

Dave Kost · Dave Kost's Garage

FUlly charges battery and throttle plate open during compression test?

fanaudical · fanaudical's Garage

Welcome to the forum. Where are you located? You may have another local Pelican willing to lend a hand.

If I read this correctly, the engine was run at one time with carbs and you have switched back to the CIS. Do you know what camshafts are currently installed in the engine? (There are CIS-specific cams which do OK with the CIS, but many who converted these to run carbs also swapped cams for more power. Carb-specific cams may create backfiring issues with CIS.)

Regarding compression - I have found compression values to be somewhat "subjective" if you have a weak starter/battery or other issues. Sometimes a motor with good compression doesn't appear that way if the starter won't crank well. I'd suspect something like this if your numbers are all consistent. Do you have access to a cylinder leak-down tester? That might give a better answer regarding rings vs valves sealing.

Avant · 10-25-2015, 04:21 PM

I was told that they are original camshafts. To the questions asked, I did the compression test with throttle wide open, and the battery is new and strong. I had the battery charger on it to make sure I was getting the right power. I also removed all spark plugs when testing, and fuel pump is disabled.

E Sully · E Sully's Garage

Time for a leak down test to see if you are losing air past the rings or valves. If there is excessive carbon on the valves they may not seat well.
Also check the head studs with a torque wrench to see if they are pulled.

Flat6pac · 10-26-2015, 03:47 AM

Just check that the head nuts are tight, don't go for torque unless you're in the mood to rebuild.
Bruce

javadog · 10-26-2015, 03:49 AM

Do a leakdown test.
Look over the CIS components and make sure they are all correct for your engine.
Verify each of the components is working as it should.
Measure your fuel pressures, make sure they are correct.
Set the CO using a gas analyzer.

Drive the car a while and see if it improves.

JR

Dave Kost · Dave Kost's Garage

Sounds like you know what your doing.

Sounds like a lean miss.

Push (hard) on the air box while the engine is running . Some time this will open up some small air leaks in the cis system and the engine will change rpm.

How does the car drive down the road?

Paulporsche · 09:04 AM

As Javadog suggests, you should check not only your fuel pressure, but your control pressure. This is very important as it confirms the operation of your WUR (warmup regulator) which controls the mixture at all times, not only at warmup. You may find your cold control pressure is too high, resulting in a too lean condition during warmup. Too lean often results in backfire into the intake.

Let us know what control pressures you get @ what temperatures.

Paulporsche · 10-26-2015, 09:06 AM

This chart shows control pressures for various WURs. Verify which one you have and compare.

Daves911L · Daves911L's Garage

Hate to be so pessimistic, but.......

Cardinal rule of engine buying should be if the receipts are not there to support it, and you don't personally know and trust the guy who did the work, assume its a rebuild core. Not many guys take a good, fresh, strong, motor out of a car and let it sit for 3 years. Those are pretty poor compression numbers. I'm betting you'll be tearing this one down soon.

javadog · 10-26-2015, 09:26 AM

You might want to start with looking at your engine type number to figure out which engine you have. That will dictate what CIS components should be on it. Lots of changes from '74-'77.

If you have a WUR with Bosch numbers ending in .017 or .033, as used in 1976-1977, then you get to figure out if the cold control pressure graph in the spec book (as shown above) is the correct one, or if the one shown in the factory service manual (with higher pressures) is the right one.

I don't know the answer to that...

JR

javadog · 10-26-2015, 09:38 AM

The black outlined area in the graph below (bottom right) shows roughly what the factory manual says it should be.

There should be a difference of roughly .6 bar between the control pressure with and without vacuum, so maybe one of the graphs shows the pressure it without vacuum and the other graph shows it with vacuum. I haven't thought about it much. You'd think they'd be consistent, but...

JR

Avant · 10-26-2015, 02:50 PM

Thanks for all the posts. I have been reading other posts, and saw some who swear by Marvel Mystery Oil as a way to clean up cylinder walls and regain sealing from motors that have sat a while.

Yesterday, I had a bit of a scare. I started the engine and got it back up to temperature so I could run another compression test. The engine drips a bit of motor oil, and while I didn't think it was possible, somehow oil made its way into the heater side of the heat exchanger. So there was a little fire in the exchanger. These are new SSI exchangers. I suppose oil made its way in through a seam?

Anyway, I think it is time to get this car driveable, and see if performance improves by burning out old carbon. But of course, this bit about oil/flames scares me a bit.

pors1968 · pors1968's Garage

With dry test 71-85 i would ckeck camshaft ajustement they open to early.

Avant · 11-02-2015, 03:38 AM

Well, after some further inspection, it turns out that the compression gauge I was using was in fact faulty. It was a new unit from Harbor Freight. I even tested it by blowing air into the unit, so I figured it was giving me correct readings. I never thought to check the hose. The shraeder valve in the hose must have been defective. Anyway, $25 later, with a new gauge ordered from Amazon, I can move on. Lesson still not learned: Stay away from Harbor Freight.

That being said, I got compression in the 90 to 120 range. I also did a leakdown test and I had strong hiss from below. This led me to find that my head stud nuts were loose, almost all of them on the bottom. I was able to re-torque them. But will they hold, or come loose again? I will try to run it today and see what kind of compression and leakage I get with the tightened stud nuts.

yelcab1 · 11-02-2015, 06:39 AM

Quote:

Originally Posted by Avant

Well, after some further inspection, it turns out that the compression gauge I was using was in fact faulty. It was a new unit from Harbor Freight. I even tested it by blowing air into the unit, so I figured it was giving me correct readings. I never thought to check the hose. The shraeder valve in the hose must have been defective. Anyway, $25 later, with a new gauge ordered from Amazon, I can move on. Lesson still not learned: Stay away from Harbor Freight.

That being said, I got compression in the 90 to 120 range. I also did a leakdown test and I had strong hiss from below. This led me to find that my head stud nuts were loose, almost all of them on the bottom. I was able to re-torque them. But will they hold, or come loose again? I will try to run it today and see what kind of compression and leakage I get with the tightened stud nuts.

Never buy anything precision from Harbor Fright. Period.
As for whether your retorque will hold, you will not know until you run a few hundred miles into it. 50/50.