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Tech QuestionsJune 2003 Tech Q & A

Answers by Technical Editor Rex Chalmers

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June Topics:

Plenum Pop | Timing Belt Change Intervals | Aftermarket Seats and Belts
Exhaust Sensor Light Reset | Engine Miss | Oil Pressure
Compression Testing | Valve Job or Rebuild?

Plenum Pop : question by Bob Rocco | 1989 Milano Verde

Yesterday, my son said his 'silver box' blew off in the parking lot, putting a slight dent in the hood. I went to investigate. He replaced it and the car started, so I tightened the six hoses. Later, I found out this is called "plenum pop." I've had two V-6's - still own one, and never had this occur. Is there a solution?
Answer : Yes Bob, there is a solution. “Plenum Pop” is caused by an overly rich condition on start-up. As with many automotive maladies, it can be caused by a number of malfunctions. First, make sure that the gas pedal is not depressed on starting. If you have to use it to get the engine going, there is something amiss with your fuel or ignition system. The most common cause of “plenum pop” that I have found is a malfunctioning cold start injector often in combination with an auxiliary air valve that is no longer opening to specification when cold. If the cold start injector is not atomizing fuel properly and just dribbling the fuel into the plenum it tends to pool and when it is finally ignited it goes off with a nice little explosion that causes the plenum to separate from it’s mounting hoses.
There are a number of other scenarios that could cause your plenum to pop including but not limited to:

  • Weak ignition spark (coil, spark box, spark plugs, ignition wire set, distributor cap, ignition rotor, etc.)

  • Fuel system related (combined relay, pump switch in air flow meter, thermal time switch, T2 temp sensor, faulty injector set, low fuel pressure, etc)

I’d give your son’s Alfa a good tune up, including cap, rotor, air filter, spark plugs, fuel filter, O2 sensor if it hasn’t been replaced in 40 or 50 K miles, treat it to a new set of plug wires and make sure not to depress the gas pedal when starting. If you still have to use the gas pedal when starting after the tune-up, chances are you have a problem with too much fuel or improper atomization of fuel. At this point, you may have to consult a professional Alfa wrench.

Timing Belt Change Intervals : question by Alan White | 1986 GTV6

Everyone seems to agree that V6 timing belts should be changed every 30,000 miles. I only drive my GTV6 around 5,000 miles a year, so it will take 6 years to reach 30k. What is a safe time interval for timing belt changes on cars that are not driven often.
Answer : To a large extent it depends on where you and your Alfa live as to how often you should change the timing belt. If you live in a dry area with a high ozone content, such as LA, it will probably be cracked and dry rotted in two to three years. In other climates it could last several years longer. The general rule that I follow is pretty much just common sense and observation. Check for cracks, cog separation, signs of rotting and for proper tension. The bottom line is that even if the belt looks OK, I wouldn’t let it go any more than four years. Changing that belt is very cheap insurance against the cost of pulling the heads and replacing valves and guides or worse.

Aftermarket Seats and Belts : question by Rob Fallon | 1972 GTV 2000

My car was often shown at concourse shows before I got it so it is almost entirely original. I improved the suspension and added better wheels so that I could take it on some nice country rides. My question, I need racing seat belts and which maker do you recommend. Do you recommend changing the seats? Adding a roll bar?

Answer : I like Sparco belts and seats. Not because they are any better than Simpson, Willians, or any other quality belt or seat. No, I prefer them because they are Italian, and consequently are of excellent quality and are designed with a wonderful sense of style only the Italians can pull off. My body never liked GTV seats, on long trips I would get horrible back pain, and lateral support is at a minimum. I would definitely go for a nice set of sport seats and matching belts. GTVs don’t always roll over very well, so if you feel the need, go for the roll bar. Looks like your GTV isn’t going to stay too original for long!

Exhaust Sensor Light Reset : question by Jeff Woods | 1982 Spider

My 82 Spider has an "EXH SENSOR" light in the display row containing brake, seatbelt, and headlight lights. At about 90,000k the light came on. However, I've had the emissions, etc. tested and there is no problem there. The conclusion I (and others) have come to is that, because California restrictions were new in 1982, this light comes on at pre-set intervals (probably according to milage). My question is, if this is true, how do I reset it?
Answer : I don’t know if your conclusion as to California law is correct, but the light is set to come on at pre-set intervals. There is a re-set button. Follow the Speedo cable up past the steering box; it then travels to a junction box where the reset button lives. You may have to drill a rivet, or cut off a wire with an impressive lead seal, or take off whatever fastener is now adorning the cover, assuming that the cover is still in place.

Take off the cover and, with an appropriate tool, I use a radiator hose tool, push the button. Presto, the pesky light is now extinguished. If that doesn’t do the trick, you may have a bad button box. I have experienced several faulty reset button boxes - then just take the bulb out. One more thing, if you haven’t changed the O2 sensor in the last 40-50 K miles, change it. Your Alfa will run smoother, and get better mileage.

Engine Miss : question by Tony Miraglia | 1978 Spider

I took my car out for a drive today and while on the freeway shifting from 4th gear to 5th at about 5000 rpm the car which had been running beautifully missfired. I backed off the acelerator and it continued to missfire with any amount of gas I gave it. I managed to limp home and immediatley checked the plugs which were fine...repaced them anyway and the car still missfires and although it runs, runs very rough. Since no one in my area knows anything about Alfas I went on line and found your site. Can you help me please.

Answer : Your question begs the writing of an Alfa troubleshooting manual and appropriate flow chart. An engine miss can be caused by your ignition or fuel system, not to mention mechanical damage. Since I don’t have the time to write that manual, I will go over the basics that you may be able to attend to yourself.

First, check for mechanical damage. Do a compression test with the engine hot, all spark plugs removed, and the throttle completely open. Next perform a leak down test. There is the possibility of a broken valve spring, which can lead to a bent valve. A good tech can pull the cam cover and compare valve spring tension without taking all the springs and cams out.

If the engine appears to be OK mechanically, check the ignition system. Check the dwell of the points, if you have an original distributor. If you have some flavor of electronic ignition, contact the seller and get specific troubleshooting directions from them. Check the plugs for burned up insulators, physical damage, improper gap, or overly rich or lean mixture. Also check to see that they are all burning the same. Check the resistance of the plug wires. The plug wire test will reveal a plug wire with too high of resistance or an open short in the wire, but it will not identify a wire that is faulty only under an electrical load. Look up the specifications of the coil and test it. Also check to see that the voltage at the coil is at battery voltage.

To check the fuel system you will need a pressure gage and a graduated cylinder. Install the pressure gage and check to see that you have the specified fuel pressure for your Alfa. Next check the manual for the fuel delivery rate and check it with the graduated cylinder. Pull the air cleaner, start the vehicle, and check for a strong fuel smell around the injector lines. The lines have been known to fracture, and the fittings have been known to loosen up causing loss of fuel pressure in that line, and consequent loss of pressure to the injector and loss of fuel to that cylinder. If all of this checks out OK, you will probably have to seek out professional help.

Oil Pressure : question by Greg Loper | 1974 GTV

The oil pressure gauge reads from low to zero. Usually low on start up, then back down. Is it the gauge? There is no operational problems. I was reading in the tech questions about the head gasket leaks and shot O rings resulting from not changing the oil often. As my car sits for months or years, the oil hasn't been changed regularly. Would the failing O rings cause the low oil pressure?
Answer : The amount of oil that leaks out of faulty O rings won’t cause a large enough drop in oil pressure to be detected by an oil pressure gage. The first thing to do is check your oil pressure with a mechanical gage to make sure that there is no problem with the lubrication system. If the pressure checks out OK, and it probably will, it has been my experience that about 95+ percent of inaccurate oil pressure readings in Alfas are caused by a bad sending unit. The other possibilities are a faulty gauge, loose or broken ground wire, power wire or signal wire.

Compression Testing: question by Mike | 1973 GTV

I'm considering buying a 1973 gtv and was wondering what a normal compression test in the cylinders would be. The owner tested all the cylinders at 140. Is this a good number?
Answer : 140 is an OK number. What is than the actual number is that the numbers be within a few percent of each other. Generally, I like to see a number somewhere between 140 and 160. A number above 160 would indicate carbon build up on the piston, valve heads or both, higher compression than stock pistons installed, or a cylinder head that has been milled. Due to differences in compression gauges, and methods used for executing the compression test, I would suggest that you arrange a trip with the owner to a mutually agreed upon Alfa wrench and have a compression test done. Remember, all the plugs must be out of the head, the engine hot, the throttles fully open and each cylinder tested for the same number of compression strokes. I like 8-10 strokes per cylinder. You can tell when you have turned the engine over enough times as the compression number will stop increasing.

Valve Job or Rebuild? : question by Peter | 1974 GTV

I have 100k miles on my very clean GTV. The only parts of the car which hasn't been renewed or rebuilt are the engine and transmission. I have all new suspension parts, master cylinders, brakes, electronic ignition, a rebuilt ingram pump and a nice paint job. The engine is a little tired. The compression in the second cylinder is 90. The other cylinders are all around 150. I am wondering what you would recommend to bring the power up. I just miss the power a previous stock GTV I owned had. Would a valve job alone make a big difference? Since I already have alot invested in the car, I don't want to cut corners here, but I would like to keep costs down.
Answer : Chances are that with 100K on the clock, your baby is ready for a rebuild. The only way to figure out what your engine needs is to perform both a compression and leak down test. If the tests show that the leakage is from a burned valve, you could do a valve job and you would add some extra life to your engine. The problem with doing that is that if the rings and rod bearings are not in the best shape; you will be shortening the rest of their useful life by doing the valve job, as they will age more quickly due to the increased compression that will result from the valve job. You also run the risk of more catastrophic engine damage. It may or may not be worth a shot.

If the tests point to rings as the cause of the loss of compression, you could just re-ring it, and/or replace the rings, pistons and liners. At this point I would strongly suggest that you do a complete rebuild. After 30 years and 100K miles of faithful service, even Alfa motors can use new seals, chains, bushings, bearings, pumps, pistons, liners, valves, springs etc. In my experience, doing valve jobs, and ring jobs on engines of this age is not cost effective. They may fix the immediate problem, but inevitably, not to far down the road, another system failure occurs, or a part fails causing even more damage. The money spent on the band-aide repair is generally lost.

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