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1991 R2500 454 suburban drive ability issue

K

Klink K5

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My 1991 2wd 454 with 4L80 sub has a slight surge (always there and annoying) at cruise speed 55-70 mph. Problem started right after I changed the ecm with a Delco re-man unit

Anyway all ignition components are new / good, cylinder compression test is good....... all cylinders over 130 psi, fuel pressure & injectors are good, timing checked , no check engine lights........ cold it has a non steady idle warming up...... runs fine under heavy acceration ........ then when at cruising speed, starts to buck a little.......

Transmission Lock up is working fine verified with snap on scanner as well. Has anyone had the egr valve so weak that it opens up and cause this problem?
 
What was the cause that prompted the ECM replacement?
 
Wouldn’t start........ fuel injectors wouldn’t fire, changed module in distributor as well still wouldn’t start then put in a different new distributor. Still wouldn’t start. Computer was replaced and it ran, chasing this gremlin ever since

New fuel pump / filter as well....... all delco electonics, nothing less......
 
May seem to me that the programming in the new ECM is either missing a parameter
 
Only thing I can think of is that the fuel pressure regulator may be bad. If the fuel pressure regulator diaphragm is old and wore out it may not be holding pressure consistently.
 
Disconnect the EGR solenoid and see if anything changes. Check engine light may come on, but the PCM won't change how its running the engine due to EGR "failure". Non-steady idle warming up? As in it fluctuates? If so, look real good for vacuum leaks. Snap on tool should tell you TPS, MAP, etc, correct? Take a look at all those values and make sure they are within spec.

Also, if you can access it without too much trouble, try re-seating the PCM connectors, making sure all the terminals are locked into the connectors.
 
Just replaced fuel pressure regulator, zero change. I had 13 psi of pressure at incoming fuel line to tbi before I buttoned it all up.

I did unhook egr valve, no change........ egr valve and regulator all done within last hour of this post.

Still bucks in overdrive / drive gears with lock up converter engaged.

Either I have 2 things wrong..... plugged stock exhaust or lock up converter circuit failure going on........ when the problem happens it’s at hwy speed 55-75 mph and seems like someone is cutting spark / fuel to engine......... 0 to 10% applied to throttle pedal.

Gonna check exhaust back pressure next
 
  • I also removed ecm and checked connections again as well.........acd# 88999179

Nice and dry......... good terminal retention

I’m wanting this thing to run perfect while it’s 2wd Before I convert it to 4wd ....... with 4x4 frame , new 4L80 4x4 trans and 241 t-case. Oh Dana 60 of course, all ready has the 14bolt ff rear.
 
If the exhaust was plugged/severely restricted, under WOT throttle it would start to lose power. You would be able to see that through the scantool as well, if the vacuum (MAP) increases under WOT.

Is there a way to trick the 4L80E into keeping the lock up converter from locking, so you can test it at freeway speed? I don't know as you'd be able to see through RPM the difference between TCC lock/unlock and surging caused by another event. I'd *think* it would be, but might appear pretty similar when looking strictly at RPM.

You'd see knock counts/timing retard on the scanner if it was retarding timing due to the knock sensor doing it's job, or not working properly.
 
I've seen a few freak out and retard the timing erratically at the slightest "knock" before...try tapping on it gently (if it can be accessed ) with a screwdriver while its idling and see if the engine misfires or has a noticeable change in how it runs...sometimes they will go sour and not throw a code..

The exhaust back pressure being excessive is a possibility too...cant let enough out fast enough to not affect the incoming fuel mixture..

A friend's Jeep Liberty he bought for his wife lost the exhaust valve seats due to partly clogged up converters,they were the type mounted right off the exhaust manifolds--the heads got too hot and it dropped at least one valve seat,he bought a low mileage used engine instead of trying to fix it..

You might be able to "disable" the lock up converter by applying the brake pedal just enough to activate the brake lights...that works on older vehicles,not sure about ones with ECM controlled transmissions..
 
So just for a test drive watching my snap on Solus pro scanner...... the knock senor working fine but when I was at 10-15 % throttle, the spark advance would go from low 30’s to 45-50 degree advance just like that...... then the bucking would occur like the problem stated earlier........ also when in drive or overdrive, the scanner shows ???? While in gear

So I disconnected the map sensor, now advance would not get above 36 degrees and no problems of a buck or miss are present..... so is my map sensor bad?
 
Well I tried a different MAP sensor and TPS because I had them around....... I didn’t gain a thing back to jumping / jerking at hwy speeds in lockup drive / overdrive......... I’m leaning towards replacing the computer again because that’s when the problem started. My scanner shows converter slip in the transmission parameters jumping from 0 to 600 rpm....... when slipping @ 600 rpm, spark advance goes from 33 degrees to 50 degrees as well. I’ve never heard of any engine running well at 50 degrees advance. Seems like a bad pcm internal ground........ stay tuned

Thanks
 
Since you have an early model 4L80E, you may want to look at the transmission internal wire harness connector. Inspect it for transmission fluid leaks or dry rot cracking. It is the large round one (about 2" Diameter) right at the trans pan (left side). When these connectors go bad, they can send improper signals to the ECM/PCM.
 
Can Can said:
Maybe a dumb question, but did you replace your wires?
Click to expand...
The entire ignition system is new / replaced with delco parts from distributor to the spark plugs. I’m using conventional ac/delco plugs as well no exotic ones.......

MSD 8.5 mm plug wires properly routed to prevent burning /misfires as well. The coil has not been replaced, don’t see the need to replace it. Has good spark.
 
1-ton said:
Since you have an early model 4L80E, you may want to look at the transmission internal wire harness connector. Inspect it for transmission fluid leaks or dry rot cracking. It is the large round one (about 2" Diameter) right at the trans pan (left side). When these connectors go bad, they can send improper signals to the ECM/PCM.
Click to expand...
Transmission was previously replaced with a rebuilt GM unit......... not by me but has the decal on the side of it. GM rebuilt......... not ruling the trans has issues yet but wiring is nice and dry on transmission electrical connections. Ran outa time to throw at it for the day. No tranny codes either.........Going to play with it again soon.


Exhaust back pressure is my next step. I have a fuel pressure gauge just ran outa time

87,000 mile truck........

Checked y pipe temps / converter temps today and all over 600 degrees with truck off checked with my Fluke infrared temp gun. All Stock original exhaust on this one, has the muffler in front of the converter........ stay tuned....... I live in Iowa so the original muffler / converter will disappear for good. Will put on a stainless borla 3 inch in / out is my plan
 
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Do you have the old pcm still? I assume these still have easily removable proms, swap the old prom into the new pcm if you think it's along those lines. Not convinced that's it. What pcm number is this, and what bcc? Bcc is four digits, usually alpha, not numeric.
 
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