Hard shift, then bogs down...lack of power
 

2000 Blazer 4x4 automatic 4L60E

Symptoms:

hard shifting in all up gears, followed by a serious power loss. The only way to get any serious acceleration, is to downshift, which works, but as soon as it upshifts, its bogs down. If I'm going 40 mph in 3rd, and want to accelerate, I gotta downshift to 2nd. Additionally, the Torque converter seems to be confused in 3rd and 4th in the 40-60 mph range. Constantly engaging, and disegaging, hunting for the right spot.


Tranny fluid dirty, took it to Aamco, said tranny will go soon, but operating fine now. They said engine was weak, which is why the tranny was needing to downshift to find power.

Took it to the dealer 3 times, first they claimed CAT was blocked, so I removed it, second they said the muffler was blocked, so I replaced that, third time they had multiple techs look at it all morning and finally declared there was nothing wrong and 'those 4.3 engines are just dogs'.

I'm still not convinced that the tranny is shot. I just drove it from 5200 ft to 9500 ft and back with no problem (except the downshift for power issue).

What could be causing this bogging after every (hard) upshift?

possibly, bad tps sensor, weak fuel pressure, in need of a major tune up plugs,wires,cap,rotor,02 sensors pending on how many miles. with out being able to look at it myself very hard to say i would check the things out that i mentioned. the plugs well tell you the story if you know how to read them.

Changed tps, dealer checked fuel pressure at idle and driving, new plugs, new plug wires, only code is cat low efficiency, since it's gone.

The dealer had it 3 times, used all their diagnostic tools.....nothing!

Changed tps, dealer checked fuel pressure at idle and driving, new plugs, new plug wires, only code is cat low efficiency, since it's gone.

142K

The dealer had it 3 times, used all their diagnostic tools.....nothing!

If you did the spark plugs and wires and it is still acting up I would continue with the Distributor cap and rotor as well. Maybe even think about getting the ignition coil and ignition module checked out. Also what did the dealer say about the fuel pressure? They may have said something that might allow War00 or someone else to help you out more.

3 things come to mind with a bogging situation fuel issue, spark issue or clogged cat. 1)fuel leak down test needs to be done to see if the system is holding pressure, if it doesnt hold than you could be looking at the fuel pump or the spider asy. does it smell bad when its idling like unburnt fuel or off smell 2) if you havent changed the cap or rotor than do soo, or the dizzy needs to be checked for play and if the gear is worn than it needs to be replaced because the the timing is retarded. Finaly the cat is clogged, pull the o2 sensors and take it for a drive and see if it's better. since you said you had a code for the cat id almost put my bets on that. simple to do your self with a 7/8" wrench.

Park & war00 are giving good leads,but I would look into other issues as well... You did not give enough history of the problem. As you know the 4.3s aren't necessarilly dog engines,but the only time I drove one at high altitude (my 87 GMC K15-non-computer controlled) it was very weak over the passes in CO!I suspect transmission solonoid problems above and beyond what the other folks have suggested-AAMCO is probably correct,the tranny does not have long to live:but they are going to say that to EVERYONE!!!! How many miles?Trans maintained? If you do have your trans re-built,take it to an independant ATRA (automatic transmission rebuilders association) shop-NOT a chain-be forewarned! Best of luck! step66

reply for war00 and step66
 

Thanks so much for the input, let me go over a more detailed history so I can get some final advice before I start replacing things.

We bought it (for my teenager) used with 139K about 3 months ago, had a rebuilt tranny from a dealer with a 3yr 36K mile warranty (at 96K-2005).

Has 2" body lift, 31"tires, aftermarket radiator, High flow air intake and exhaust.

Car lives at 9100 ft (mother), and 5200 ft. (me). Car spent the first half of it's life in Souix City, IA, and the second half in Boulder, CO, with many road trips to Utah and some hard driving, I suspect.
__________________________________________________ _______________

The following were said to be done by the previous owner (we believe him)

new plugs
new wires
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we did the following after purchase.

New brakes, and rotors
new shocks
new serpentine
new tps
new fuel pressure sensor (on top of fuel pump)....funny story here, we got the code for this sensor, and when we checked it out, the ring holding the fuel pump into the tank was disconnected, and the fuel pump was floating out of the tank, but it still ran...we reconnected the ring and the pump. we found the new wiring harness for the fuel pump in the back, so it must have been replaced improperly.

cleaned MAS
cleaned EGR
new gas cap
removed the catalytic converter (replaced with straight piece of pipe)
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We have two outstanding issues.

The left front wheel bearing is bad, and the wheel sensor is not working, so the ABS light is on.

The Check engine light is on because the cat is gone (catalyst below threshold)
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The original issue was the hunting the Torque Converter (TC) was doing in 3rd, and 4th gears and the lack of uphill power. That's when we took it to Aamco. They originally said the brake switch was inoperable, causing the TC to get confused. We remembered that the previous owner told us he pulled the ABS fuse, when we replaced it, and tested the transmission, it was OK, except the burnt fluid, of course. (That's probably what burnt up both transmissions!) The Aamco owner said it was a matter of time before the tranny died, the rebuilder said maybe, but the tranny was operating fine now, and it was probably a weak engine.

So off we went to the GM dealer. They said there was backpressure before the cat and it had to go. We took it to our exhaust guy, who removed the cat, and it was empty, I mean nothing at all in it at all. And he also told me that no one had checked the back pressure at the dealer, as no o2 sensors had been removed. Anyway, back to the GM dealer we go, they had every tech in the shop scratch their heads over it and hooked it up to every diagnostic tool they had and could not find anything. They declared it a 'DOG' and reminded me that the transmission was bad and sent me on my way.

The reason we took it to the dealer in the first place, was to get a professional confirmation that our spider needed to be replaced, but they said it was not leaking.

So here I am.

Nothing there about fuel delivery, fuel pressure is critical on these engines. Check fuel filter for restriction, then check fuel pressure and leakdown: key ON, engine OFF, fuel pump running, pressure must be 60psi to 66psi and must remain above 55psi for 10 minutes after the pump shuts off. The results of these tests will determine if the fuel delivery system is working properly. BTW, no such thing as a "weak" 4.3L, a technician with weak diagnostic skills is another story ;)

Hook is spot on about fuel pressure needs espescially at high altitudes!

I'll report all fuel tests Monday...Thanks

Fuel pressure test reports:

Key on/engine off 59-60psi (hits 60 as soon as key turns, then immediately backs off to 59 and stays)

Engine idle 54 psi, constant
engine rev 56-60psi
turn engine off 61psi

leakdown tests as follows...

first test...

minutes / psi

1 / 59
2 / 59
3 / 58.2
4 / 56.4
5 / 56
6 / 55.8
7 / 52
8 / 53
9 / 52.5
10 / 53

second test immediately after first test

minutes / psi

1 / 60
2 / 60
3 / 59.5
4 / 59
5 / 58.5
6 / 58
7 / 57
8 / 57
9 / 56.5
10 / 56


Any advice?
What does it mean?


Note....twice, after revving the engine, and then turning the car off, the pressure faded fast to the low 40's....I tried to duplicate it, but it held firm the next few times.

Oh, btw, we replaced the fuel filter earlier, just forgot to mention it.

Couple of things....
GM does not publish any "engine running" fuel pressure specs for this engine. Just for the record, 54psi at idle is pretty common. ;)
Key on, engine off, pump running: Your reading of 60psi is the minimum allowable. Could just be gauge error, (reading low) but your running pressure is normal, which gives no reason to question the accuracy of the gauge. Leakdown is acceptable. The only other test for fuel delivery is pump volume. It should deliver a minimum of one quart in 30 seconds, unrestricted. In other words, straight out of the pump into a jar etc. If you decide check it, you'll most likely find it to be very close to the minimum, just like prime pressure. Bottom line, I doubt fuel delivery is the problem.

Have you checked camshaft retard? It must be checked with a scan tool capable of accessing it, (obviously) ;) Not all scan tools can access it though. If cam retard is off enough, it can cause a lack of power, misfires, and cause transmission shift points to change, along with other "issues". Spec is zero degrees + or - 2 degrees and it will not set a DTC and turn the SES light on unless it's off by more than ~25 degrees. For example, if it's off by 20 degrees in either direction, the engine will run, but engine and transmission performance will suffer, and no SES light!

OK, thanks for the input. What tool measures camshaft position? It it a sensor? Do you have to remove the motor? I called my local mechanic, he said I have to find an engine building shop.

OK...maybe a fuel volume issue.....did 4 tests, 30 seconds each, got a little over 1/2 quart in 30 seconds.

test 1 600ml gas cap off
test 2 620ml gas cap off
test 3 650ml gas cap off
test 4 650ml gas cap on

anyway, each is about 2 1/3 cups, just about half of expected value.

Could you please verify volume expectations.

Thx...

Camshaft retard is checked, (observed) using a scan tool. Plug in the scan tool, select the correct menu on the scan tool, idle the engine, snap throttle up to 2,000 RPM and back to idle, and read it. If it requires adjustment, loosen the distributor hold down bolt and rotate the distributor. The factory hold down bracket will allow ~10 degrees of total adjustment. If more is needed, remove the distributor, remove the OE hold down bracket and toss it, install a SBC, (small block chevy) hold down bracket. This modification makes camshaft retard fully adjustable. Most auto parts stores stock the bracket for under $5, made by Mr. Gasket... high performance chrome plated ;)

Fuel pump volume: Actual spec is 23.28 ounces in 30 seconds. Sorry, I was off a bit ;) Your average is ~21.5 ounces in 30 seconds. Volume is as expected, slightly low. Low volume will cause fuel starvation to the engine with the same symptoms as a restricted fuel filter: lack of power on hard acceleration and no SES light. Take into consideration that battery voltage to the pump is higher when the engine is running, which will increase both maximum output pressure and volume, it would be a real stretch to say this is the problem. We'll keep this in mind, but for now, check camshaft retard.

What menu item should I choose on the scan tool. Will any scan tool have these options, or do I need one with more options?

I may sound like I know what I'm doing, but I don't.

The el cheapo scan tools, ($300-$500) can't access it. Most any bi-directional scan tool will access it, but they start upwards of $1K for a decent used one. There are software programs you can install on your PC or lap top that will also access it. Better off finding a shop that is familiar with what camshaft retard is, what it does, and how to correctly adjust it. With the proper scan tool, (or software) it can be checked, modify the distributor, and adjust it, all in about 30 minutes.

Hook has it covered, as usual. I'd really look at that CMP retard.
Another indicator of low fuel delivery would be actual driving under those conditions especially at 9000 and watching fuel trims. If it's going lean at the point where it's bogging, and/or when it downshifts might be good to get a different pump.
Has the hard shift been fixed? Confused.

What I really think needs done is eliminate subjective nature of 'low power'. Help if another blazer could be test driven at same conditions. My immediate thoughts were of how aggravating it is to me that mine downshifts when I press 'increase Cruise Speed'. My Bird accels slower but rarely downshifts. 400 lbs lighter, sure but also 1/2 liter less torque.

Here's something interesting I found while researching {Yes I do that.. a lot when folks are obviously trying to help themselves}
Lose power going uphill. Engine rpm's wont go above 2,000 and no down shift - Page 2
- it's a Toyota has no power and it WONT shift when called on but otherwise pretty similar. Thing is, that should have been reflected in the long term fuel trims

All that aside, what hasnt been addressed is the possibility that the cam timing is off. I mean the cam itself. The main shortcoming of the GM method is the PCM has no idea where the cam actually is.

OK....she's not retarded (the blazer, I mean)....took it to a shop, did CKP correlation procedure, said it checks out for cam / crank alignment...

Next...!!!???

Is it possible that the 31" tires, brush guard, roof rack, and tow hitch are weighing it down?

Maybe I should rephrase that.
How does the VCM know when cam driven valves are opening and closing relative to piston travel?

My theory and supposition is.. it doesnt! If it does, please explain how. Only possible way I can think of is the MAP... and that's a reach because of plenum capacity.

Again.. I am not saying that's the problem, but if I accept that the engine has no power to increase speed from cruise, then it must always be considered the same now as it was in the past, before electronic systems.

When they say, "CKP correlation procedure", (incorrect terminology) I can only assume that they did the crankshaft position sensor relearn, which is NOT camshaft retard. Were you there when they did it? Did you see what they did? Can you describe it for me? This goes back to the comment I made earlier about "weak diagnostic skills", I sure hope this wasn't at a GM dealer ;)


Diagnostic trouble code P1345 indicates that cam/crank correlation is incorrect. Checking "cam/crank alignment" could mean all they checked for was the presence of P1345 in memory. If P1345 is not in memory, cam crank correlation is within ~25 degrees, in other words, it's "in the ball park". But that's a pretty big ball park. After snapping the throttle to 2,000 RPM and returning to idle, what is the actual reading of camshaft retard? The scan tool will display camshaft retard in positive or negative degrees, (it will be less than ~25).

When these things are running right, they can easily pull a 3500# trailer, so I doubt your modifications are causing the lack of power. If the PCM hasn't been flashed for the larger tires/wheels, it will affect shift points and how smooth or hash it shifts. It will also affect your speedometer and fuel mileage, but not the lack of power.

By any chance do you have an oil type air filter ie, K&N?

OK...

PCM not reflashed
Have K&N high volume dry air filter

Called dealer, they had no idea what I was talking about, called another local shop. They have the Snap on computer, and looked through the menu.....nothing for camshaft retard.

They went to Alldata and pulled instructions for distributor replacement. Looks like the procedure covers the cam/crank/valve/piston correlation. He said he could do the the distributor alignment in an hour.

Is this correct? , or advised?

If this is wrong, could you give me the exact terminology. Nobody seems to know what camshaft retard is.


btw....how/why could the camshaft get retarded?

Wallace.. see if you can find a drawing of a Timing chain setup.
First, if a timing chain is loose and jumps a tooth, the cam will be retarded in comparison to piston travel. That isnt real likely unless the timing chain is pretty loose... but shouldnt be ruled out.

But there's other ways, too and they relate to the mfg process and a bad part slipping through.. thus causing what the numskull mechanic's reference to a 'dog engine'.

The slot in the crank snout might be cut wrong. The slot in the primary sprocket wrong in relation to teeth and index {wouldnt think so} The holes and index pin in cam, wrong.

Or the thing just set up one tooth off, in the first place.
Remember.. the dist is set up to the position of the CK position sensor and piston TDC, and doesnt care what tooth in the cam it sets in.

all conjecture

Wallace, are there any shops around there that know what they're doing and how to use their equipment? Wow, I'm beginning to think they're all numbskulls. Camshaft retard is the synchronization of the crankshaft position sensor with the crankshaft, and the camshaft position sensor with the camshaft. The P1345 will set if the falling edge of the cam sensor signal leads or lags the falling edge of the crank sensor by more than 26 degrees. Camshaft retard has a substantial affect on ignition timing and can greatly reduce the longevity of the distributor cap and rotor if it's not within specs. When both sensors are synchronized, the PCM also uses the data for identifying which cylinder/cylinders are misfiring. Keep looking for a tech and a shop that understand it.

OK, I'll look for a better shop next week, and report back.

I live in Boulder, CO...they hate cars here. Although they have the coolest Ferrari garage and race set-up shop here you could imagine. Surgical gloves, tiny instruments and magnifying lights. Maybe I'll ask them for a referral.

Thanks