This is page is brought to you by JJ and hosted by Steve. We will start by saying that we are not the foremost authorities on the 3.4L Twin Cam V-6. But JJ however has collected a pile of info on this engine. From the owners standpoint, a GM autotech, and weekend racer. If anyone who may read these notes has something to add, or even correct, it would be appreciated. Sharing info amongst us 3.4L DOHC owners is going to only help us all. The reason JJ wrote this and I host this site.
The 3.4 was concepted as a V6 version of the Olds 2.3L "quad 4". The quad motor went into production in 1988, and that also marked the start of the V6 design program. The 3.4L DOHC was actually the forerunner of many current motors. The Cadillac Northstar 4.6L 32 Valve V8, and later the Aroura 4.0L 32-Valve V8, were sons of the V6 program. Now we have expanded into the 3.5L V6 and to a degree, the OHC Truck V8 small-block. Most of us wouldnt put that as important, but the lessons learned on the Quad 4 went into improvements for the 3.4 V6, and subsequently, the Northstar and Aroura.
The 3.4L DOHC motor was designed with a CHEVY name, and was slated to go into the W-10 body cars, namely the Lumina, Grand Prix, and Cutlass. Buick opted to go with their 3800 Series I V6. Target date was 91 model year, as Lumina Z34's, Grand Prix GTP's, and Cutlass Supreme Internationals. In that order. A funny thing happened on the way to the assembly line....When it was orginally concepted , the little Quad was pushing almost 200hp on 4 Jugs. So the V6 needed to have some more power. Now GM at that time was not know for a good FWD Transmission. Even the rocket 180 HP quads in N-body cars came with 5spds only. No auto was offered as HI-PERF, quite frankly because they wouldnt live behind the motor (remember we have to warranty the car if it breaks). Well, the GM engine gurus went to their counterparts at GM Hydramatic, with a challenge. Build a FWD auto trans, that will take 275 HP. You have 2 years to be in production. In a car. Warranty and all. Hydramatic said, "no problem". Meanwhile, the 2.8L V6 had grown to a 3.1L and was currently in use in the W-10 cars. The 3.1L was redesigned as an overstroke version using a cast iron block. To this went the best that GM had to offer, remember we're building a 275HP FWD muscle car. Forged crankshaft, forged rods, forged pistons, balanced lower end. Revamped oiling system with high volume pump. An oil cooler. On top, a pair of computer designed 4 valve per chamber aluminum heads, with hemispherical combustion chambers. Cross-Flow intake and exhaust ports, designed for maximum flow at high RPM. Next, add FOUR, count 'em FOUR camshafts, each controlling 6 sodium-filled lightweight valves. Dual dampner springs rest under a "bucket"type cam lifters, for instant and precise valve timing. The cams were supported by 4 journals each, providing durability along with the least amount of valvetrain mass. The cams were driven by a cogged belt, further reducing mechanical drag of the motor. The exhaust was cast iron manifolds, large by any standards, with a single high flow catalytic converter using 3inch downpipes. All 3.4L cars used a dual muffler system, which not by coincidence, is mathematically perferct in diameter and distance for performance applications. The intake was a tuned tunnel ram, with the early builds using NO mass air flow sensor, and 94 and later ones using one. Add to this combination Fuel injection, digital EGR, and it was by far, the BEST motor that GM engineers could build for its application. Emission certification verifed an honest 281HP on the sheets. Emission 7000RPM screamer. This was in early 1990, January I believe. Transmission? Anyone? Hydramatic had its own challenges to conquer. The THM125 3 speed was being redesigned every year, to combat the next weakest link in the unit. The THM 440T-4 four speed auto debuted a few years earlier, but was prone to valve body and erratic shifting problems. The 440T-4 also had this quirk of self-destructing if the owner happend to get stuck in snow. Not much to build from...Hydramatic had its hands full on this job. An all new FWD O/D trans was needed. Many prototypes were tested at GM proving grounds, in believe it or not, a V8 FWD Camaro. Each version had uncovered problems, aka "The next weakest link". Time was fast running out. When it came clear that the all-new trans wouldn't be ready for production by mid-1990, there was only one thing to do. Redesign the 440-T4 as best as they could. Hydramatic went to the market with their finished product just days before the deadline. Will it take 275HP? NO! Will it take 250HP? NO! How bout 225Hp? Maybe. GM engine ground was peeved! All this effort, just to be cut down at the flywheel....225HP? I want 275! Well, the rest is corporate decision making at its worst. Cut the horsepower of the 3.4L to 200 with an automatic. You can have 210 on a stick. Makes a guy want to cry, don't it? Well, warranty concerns led the list of "why" and there's no way around it. The redesigned THM 440-T4 was designated the THM 4T60E, with an early RPO code of MXO. Internals were beefed up, a heavier drive chai, a better pump, and best of all, Electronic shifting! Now, instead of finicky hydraulic valve body, all shifts and timing ere controlled by the ECM. The same one used by the motor. Big, beefy driveshafts were installed into the W cars to take the power. (on a side note, I have never heard of one breaking) By this time, tooling up was underway for the '91 model year. While there were a supply of motors, the trans were still being built a few at a time. Delco electrionics solved the horsepower problem by cutting the fuel delivery and spark advance curves of the motor. This weakened it to a 6250 shift point, and 6500 rev limiter. Free-reving was limited at 3000RPM. The first 3.4L DOHC engines were spoken for by CHEVY. After all, it was their project. Stick models didnt get orders, as most dealers preferred the auto trans models because of sales. Add to the problem, af HEFTY price tag on the motor combination, adn they were a tough sell as lot inventory at the dealers. The trans shortage had eased by Feb. of '91, and finally some units were released to Pontiac for production. These facts conclude that '91 models are scarce, some Z-34's, a lot less GP's, and virtually no Cutlass. 1992 was a carryover year for the 3.4L option. Pontiac changed wheel cap centes. This was the only visual change from the '91 cars, save the VIN of cours.
The engine used a speed-density program for the ECM/fuel injection. This consisted of a MAP sensor, a BARO sensor, ambient air sensor, throttleposition ensor, and of course RPM. Cubic inch displacement X rpm, barometric pressure, and air tempurature divided by the throttleposition and engine vacum (load). Witout fail, the computer (ECM) will deliver the same amount of fuel for a given set of input parameters from the sensors. There is no substitute. Spark timing was calculated the same way (an algorithm) based on sensor input. This is very handy to understand when you want to modify your DOHC to do more. The 3.4L works on a multiport injector arrangement. Early year 3.4's did not have a cam sensor in the front valve cover. These are MFI cars. The injectors are fired three at a time, with the logic that one of the intake valves will be open. The other two will puddle the fuel on top of the valve until opens. Starting in 1994, most 3.4's went to SFI. This reduced emissions and allowed for even learn injector rates, atmoization was improved. SFI bascially means that the charge of fuel is fired at an OPENING intake valve, so that air flow will charge the fuel as a vaport. SFI cars are a little touchier to injector rates then MFI cars. The distributorless ignition system firs added/waste spart method. It would take a volume of reference to cover it fully, but just know that it is operated from input from the crank and cam sensors. Picture two companion cylinders (one coil pair) spark leaves the coil on one tower towards the weak cylinder, (the one on the exhaust stroke). The spark jumps the plug gap from center to ground and then travels through the engine block ot its companion, (which is on the power stroke) and jumps the plug gap from ground electrode to center electrode, then back to the coil tower, completing the circuit. A big circle. It reverses every crank revolution too. Spark timing is controlled by the ECM, whcih calculates which coil pair to fire, and how much timing advance or retard is needed at the moment. The intake manifold shape changed every year after 1992. Why the different manifolds? The early ones howled at 2100 RPM in drive, the later ones changed for size constraints in the car, and there was the switch to the Mass Air Flow system, whcih added another variant. All will bolt to the lower intake, but not all will accept the other various equipment needed. Exhaust manifolds are pretty good for cast units. I cant even imaging fitting headers to this beast, but you dont need to. I have run backpressure gauage on my car, and found no significant problem save the converter itself. Al converters are restricitive, but the exhaust is low on the list of places to get more zip from a 3.4. And now that the background is covered, its time you get teh mechanicals of the DOHC 3.4L "Hemi" Hope your not too bored yet.
Think of it as a MG Midget...maintance, maintance, and more maintance. I cannot stress enought that this is NOT a car to give the kids going to college. The 3.4L takes frequent, extensive maintanence to assure its durability. All strange noises should be investigated immediately, as you may soon be walking. Maybe you arent mechanically inclined. That's ok, as long as youu can work with assistance. But if your're the type who doesnt want to spend money on frequent maintance, sell the car. It will bite you!!!Last I checked a motor from GM cost $4600.00. Used start at $1500.00. If you can find one. Oil changes....complex and incredibly long oiling system of the 3.4 is very touchy about dirt. Aluminum cam carriers, valve lifter buckets with only .010 margin, and about 6500RPM thing, you gotta change the oil and filter often. I use and recommend a FULLY SYNTHETIC OIL for this engine. Yes, it cost 5.00 a 1/4. Think about the price of a motor. That makes it better. The long oiling system is the reason why the oil pressure flucates so much while driving. GM says 15psi at 1600 RPM is acceptable, that aint alot, but consider it has a lot to do. Typical readings are 40 PSI at 1600RPM, and 15-20psi at idle when hot. Use a 5W-30 grade. There is no benefit to heavier oils, and they may be harmful if oil pressure gets too high. Fuel filters need to be changed at least once a year. Forget the 30,000 mile thing. One bad tank of gas will plug it. GM fuel injection systems are recirclating, meaning the fuel passes through the filter faster then it is used by the motor, with the excess diverted back to the tank. Nominal pressure is 47PSi. A partial restricted fuel filter will casue odd power loss and it will harm that pricey fuel pump in the gas tank. Change it 15-20,000 miles, or once a year, whichever comes first. Air filtes are critical. Since the early units are speed/density engines, their is restricated air flow. Clean the filter at every oil change, or insist that the kid at speede-lube to do it for you. K&N is ok, just make sure you dont install it and forget about cleaning and oiling. Its ok to buy cheap air filters too, just change them more ofter. Coolant and hoses need good car too. Being a bi-metal engine, glycol type coolant tends to become acidic with age. This then eats aluminum, head gaskets, and causes localized overheating, which leads to cracked cylinder heads. Hoses to be concerned with are those at the oil cooler, which get oiled everytime the oil filter is changed, and the coolant pipe that runs down the RH frame. These pipes are exposed to road spray, and will rust and leak within a few years. Check near the attaching brackets for bubbling rust. When replacing this pipe, use spray undercoating for a lasting repair. Spark plugs and wires are the weak link in any GM ignition system. Due to the variable dwell circuit of GM HEI, excess resistance in the secondary ignition ciruct will fry ign coils and modules. Thes problems usually start as a buck or miss felt in high gear at low speeds, say in OD at 45 accelerating uphill without a trans downshift. I use split tip spark plugs in my DOHC, NGK, Splitfire and others have such a plug. It is a performance advantage with this type of ignition system. OE wires are ok, as long as tey aer in good shape. Powered looking "mooneyes" along with the wires where they pass near a piece of metal indicates the insulation is getting weak, and its time to change them. That brittle split loom that covers the wires is important too, if it cant be reused, go buy some more and use it. Throttlebody, throttle blade, and intake system cleaning should not be ignored. A coating of oily "fuzz" builds up on the surfaces, adn can caue problems with stalling, and even in bad cases a check engine light. I use a throttlebody cleaner spray on the inlet, and GM top engine cleaner on the internals. (Do not spray the MAF, or you will be buying a new one) On MFI cars, this is especially critical to use a liquid cleaner injested while the engine is running to break up and clean the intake valves. Excess carbon build-up will soak up the fuel sprayed at it by the injector, and cause a lean condition. Keeping the back side of teh intake valves clean promote fuel distribution and is a major performance tip. Timing Belts. Priced one lately? I told you this was an expensive motor to maintain. This is one area where it gets REALLY expensive if you dont. Timing belt arrangement uses two idlers, a tension/idler, the 4 cam sprockets, and the drive sprocket. Unless you are pretty good with a wrench, I wouldnt recommend you try to change one. There is reason they want 500.00 to do it at the garage. Bad news is, when they go, you stop. There are many DO NOTS to the timing belt setup. I add these so you dont make an uniformed mistake. Anybody with a 8mm socket can inspect the timing belt for extreme wear. Just take off one of the plastic cam covers and look. If there is any fraying of the belt, or a bunch of black hair inside the cover, change the belt NOW. If you do it yourself, under no circumstances remove the bots from the camshaft sprockets. These sprockets are not keyed, and cam timing is lost to the tune of 700.00 and a tow to the Pontiac dealership. More so, the factory timing marks are done in yellow paint pen. It can and does wear off, leaving no reference marks to time the cams with. This then requires removing the intank manifold, valve covers, etc., and some nifty GM special tools to reset the deal. The idler pulleys are wear items as well. It is a plastic pulley, and is usually what caused the belt to fail. The replacement ones from GM are a bunch better then the OE ones were. Replace them once, probably won't have to do it twice. And about that belt...it is a highly saturated nitrile belt. Don't get a cheap one either, go to GM or Gates for this one. Around 50.00 is right price, but here's another tip. Due to the lenght of the belt, and the way it drives the cams, it does stretch a little with high mileage. This means that while cam timing on the front two cams is close, the timing of the back two cams will be off. The performance edge is noticeable between 20,000 timing belt and a new one. Cam timing, it is very important. I am a tech by trade, so changing the timing belt every 20,000miles is what I do. They sure wont go the 60,000 the factory recommends. If you race your 3.4L, or expect tip-top performance it can deliver, keep a fresh belt on it. One final work, while the 3.4L is not designed as an interference engine, it has happened that when the timing belt breaks or slips, the pistons can hit the valves. ther is only .010" designed clearance between the valve at full lift and the piston at top dead center. That is how my GP was when I got it. 6 bent valves, timing belt broken. That God I didnt have to pay the labor on that mess...One more area to inspect often is the oil pump drive. This is located on the rear (LH) of the motor, under the throttlebody. While almost impossible to see from up top, if it starts leaking oil it will leak oil VERY FAST!!! The oil pump drive is a leftover from the 2.8/3.1 design. It is where the Distributor use to go. With the advent of DIS, the dist is gone, but the gears and oil pump remain. This is sealed with an O-ring along. On the other side, is a pressureized gallery. When (not if) the O-ring breaks, engine oil will leake under whatever oil pressure is available. In extreme cases, a quart every 100 miles is possible. More bad news. to replace the .29 O-ring, the upper half of the motor must come off. Replacement requires cylinder head removal. Only good part is if this happens, you can replace a bunch of other parts with virtually no labor...its already apart. And the final think on the subject of maintaince, the transmission. I have put a few hundred passes on my 4T60E on the dragstrip. I race SCCA autocross regularly. I drive at high speeds. I live to beat someone from a stoplight or leave them halfway around the on-ramp. My GP gets driven hard, if not harder then it was intended. It now has 174,000 miles and will run with any 3.4L out there. The trans has never been apart. I am a firm believer in changing the trans fluid and filter every 20,000 miles. Flush it if needed. The biggest enemy of the 4T60E is dirt and oxidized fluid. The tans and filter/cooling system is sufficient to take road racing and autocross. It will not take neglect. I can sympathize with anyone who hates to spend money on a car. I ain't rich either, but I do know that a few dollars of maintance is worth a thousand saved in major repairs. I have had my GP for about three years. I like to drive, and have put 100K on the car. I have no fears that the car will fail me, even though the high mileage would send most into a comma. Bottom line is, that if you do own one of these cars, you have something that can last, can be driven hard, raced, and still get the groceries on Friday. It can be done. I am doing it.
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