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77 Trans Am runs rough sometimes. HELP
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jyank
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Sep 5, 2016, 1:49 PM
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77 Trans Am runs rough sometimes. HELP
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I have a 1977 Pontiac Trans Am 400 6.6 with 60,000 miles. This summer the car has started to show its age. I had it since new. The car will run fine for a day or two and then it appears not to be hitting on all 8 under a load. Like a miss or moisture in the fuel. Then the next day it may run good. When it is running bad sometimes it clears up after its driven a short period and sometime it won't clear up. Like today. Its hard to explain in writing. This issue just started this summer and I never had anything like this ever before. This year I replaced the plugs, wires, cap, rotor, coil, ignition module and pickup coil. Car ran good after all that was done. Also replaced the carb with an exact number guadrojet. The issue was there before I changed carbs. A few days ago we put on a new fuel tank due to a leak at the top of the tank. Also had to replace the right exhaust manifold due to a crack. It appears the problem I am having started after the exhaust manifold was replaced. Plugs, carb and fuel tank were done after the manifold. Compression test also all good. Myself and three guys in the shop can't figure this one out. Needless to say I spent a lot of cash on this car and its still not right. Please any ideas to remedy this will be more then appreciated.
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kev2
Veteran
Sep 5, 2016, 2:07 PM
Post #2 of 9
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Re: 77 Trans Am runs rough sometimes. HELP
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basics..
Being a classic I am guessing you are choosing quality AC Delco parts?
you have spark at all 8 cyl, all plugs (old ) looked good same color?
these were a vac nightmare, use the old school carb spray trick.
look at choke pull off, EGR, PCV, maybe selectively close a vac line one at a time.
I keep thinking of the exhaust manifold as the beginning of issue - is there a EFE valve with a vac line? could valve be sticking closed?
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Tom Greenleaf
Ultimate Carjunky
/ Moderator
Sep 5, 2016, 3:03 PM
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Re: 77 Trans Am runs rough sometimes. HELP
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Wow - Even these well cared for an low miles can eat timing CHAINS. Cam gear is almost certainly nylon and doesn't age well on the Chev. 400 and lots worse on the real Pontiac block if so. Just look at water pump and you'll see which. Not sure as around then I think could be either!
How to tell? > Set timing to TDC. Turn crank bolt while watching rotor with distributor cap removed and turn till you know chain is tight one side of it. Mark you spot turning that way till TDC. Now turn the other way SLOWLY watching distributor rotor to turn while turning crank bolt and see how many degrees it turns crank before rotor at distributor turns.
3-4 would be perfect almost new. 5+6 degrees you are at the end. 7+ it's loose enough to jump back and forth usually when shut off so runs perfectly or rough next start up.
Common to OE any of them. Aftermarket even all metal on a Pontiac block some only still made it 40K for lack of lube I think in those blocks for the chains.
Oh my - forgot but think maybe the real Pontiac you could touch the chain with fuel pumps removed not the Chev Blocks,
T
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jyank
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Sep 5, 2016, 3:14 PM
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Re: 77 Trans Am runs rough sometimes. HELP
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All old plugs looked good same colour. Spark to all 8. New pcv. Don't know if egr was checked. will do. Vacuum lines were all checked and some replaced. Choke pull off fine checked when rebuilt carb was installed. Changed the right manifold. EFE valve on left side. Now that you mention it that was checked and is working. One of the guys at the shop thinks its the fuel pump. Did some research and bad fuel pump systems sound like what I am experiencing. Also car starts great. No hesitation.
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kev2
Veteran
Sep 5, 2016, 4:00 PM
Post #5 of 9
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Re: 77 Trans Am runs rough sometimes. HELP
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disable the EGR for testing - just remove and plug vac line to valve...
fuel pressure is low - by todays standards- I would suspect it would first show up at start and WOT you would likely sense that... BUY FP is not the hardest test.
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jyank
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Sep 5, 2016, 7:59 PM
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Re: 77 Trans Am runs rough sometimes. HELP
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Thanks. But how can timing jump back and forth. I thought when an engine jumped time it was permanent until repaired. The car starts great. Idles good. All the time. When the issue occurs its under load or acceleration and varies in intensity. Feels like a misfire or bad gas. It appears when its really bad it is when the car is cold. Most times it clears up in a matter of a few minutes to a few miles and is good for the rest of the day or days. Today it didn't clear up. Another thing, at times the car appears to surge not misfire. Look at the tac and the RPM's drop and come back up.
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Tom Greenleaf
Ultimate Carjunky
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Sep 5, 2016, 11:53 PM
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Re: 77 Trans Am runs rough sometimes. HELP
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Quote">> But how can timing jump back and forth.<<"
They just can. Forces when you shut down are quickly opposite from running and starting back the other way. At first you notice usually a quick pop thru carb - backfire and it runs the same for the run then can jump back so testing will test right or wrong.
Any GM V block engine is subject to this a bit more than some others.
Olds and Pontiac blocks did it more than others - can't say for absolute sure. This car "Trans-Am" could have come with an Oldsmobile Engine! Doesn't look like it searching is out as they only called them GM engines not by division. It would help if you knew the C.I.D. not liters of displacement. The Olds was a 403 the Pontiac a 400 both list as 6.6 Liter.
You have 60,000 miles and all the time passed as well. OE would have been nylon and some came thru without from new and used metal only.
The nylon plain wears teeth down evenly and smoothly or cracks sometimes whole teeth missing or a small section.
The can run like new then act up back and forth.
It may NOT be the issue right now but will be. You need to know which engine you really have.
Memory serves me that model years 77, 78, and 79 were the last possible real Pontiacs but already putting in other division engines. Pontiac did NOT sell a V8 in model year 1980 TMK just Olds and Chev blocks survived.
Only Cadillac and Corvettes didn't mess with division swapping. Rare cars now and you need to know.
Any which way you will not use one and survive without doing a timing chain job or engine will do erratic running for years or quit all at once and make itself obvious.
YOU are rare in that you bought such an old car now, NEW! Could find yourself shocked at what this could be worth and you will be asked what engine division the engine is sometime and need to know,
Tom
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jyank
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Sep 6, 2016, 12:08 AM
Post #8 of 9
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Re: 77 Trans Am runs rough sometimes. HELP
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Tom , love the cat. have three myself. The engine is a 400 cid Pontiac. As far as worth it is price less to me.
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Tom Greenleaf
Ultimate Carjunky
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Sep 6, 2016, 12:49 AM
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Re: 77 Trans Am runs rough sometimes. HELP
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I do believe especially if you find letter "K" as the eighth digit of vin it is in fact a Pontiac. GM was in turmoil knowing that regulations were coming for both fuel economy and emissions fleet wide - average of all vehicles sold and was dumping in what was available and was in fact sued and lost over it. All you got was a 100,000 mile total engine warranty if you didn't get the division you wanted and thought you were paying for and all was fine as all were OK engines each with quirks unique to them.
That's the history. Only Olds blocks and Chev blocks would survive. Later Cadillacs used the small block 307 Olds in then what they re-introduced as their full size boat called the Brougham/Fleetwood not just the DeVille name which was FWD - the games went on for another decades so GM just said we make GM engines so take a pill folks!
I can feel you love the car - I'm a nut for certain things I've had for ages or at least a history on. Can't tell you how many vehicles that would include and just can't keep them all - have to use up something.
We'll get there with your car. I was just pointing out how common the timing chain thing was. These and others not special cars like this one if they needed water pumps by about over 60,000 or just maybe over 5+ years would ask folks - I'm almost there with a water pump and these eat timing chains which cost nothing really so want to do it now or all this over again as it's NOT going to last!
I need to put my mind in perspective how much time this is gone by as these problems were happening within 10 years of these being NEW not this far out so tricky for me you haven't already had an issue?
This is just nuts and bolts of the design not electrical, computer anything but will throw off the whole show and like a b*tch test out fine eluding discovery.
Again - think about just doing that job pro-actively and with this car would take super care to keep painted parts removed do it appear as untouched as possible, painted as OE should be Pontiac Blue, Chev was orange didn't find that for this but did the Olds block.
Good luck and if you have the space and energy keep this as long as you still love it as it would be very hard to find another with the history already which really catches my eye for this thread.
See private message about cats not relevant to this thread----------- Tom
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