39Buick Posted March 25, 2017 Share Posted March 25, 2017 Link to comment Share on other sites More sharing options...
Ronnie Posted March 25, 2017 Share Posted March 25, 2017 Welcome to the forum Jim. It is hard to guess what the problem might be without experiencing it. Do you think it is the engine? I would try this to help determine if the problem might be the engine: In a save place like an empty parking lot, start the car and put it in gear. Hold the brake tightly and apply LOTS of throttle to put a load on the engine. If the bucking starts with the engine under heavy load while sitting still you can probably rule out the transmission. If the engine doesn't start bucking when under a load I would tend to agree with your mechanics that the problem is with the transmission. Link to comment Share on other sites More sharing options...
kdirk Posted March 26, 2017 Share Posted March 26, 2017 Thi sounds like an issue I had on my one 88 coupe a couple of years ago. I thought it was transmission related at first, and changed the vacuum modulator which rendered a very minor improvement, but didn't fix it. Lived with it a while but then my ICM died. Replaced it, and lo and behold the bucking/stumbling stopped. Turned out to be the ICM all along. The one that died was only 2 years old and had apparently been defective from the start. I'd never have guessed that the ICM would cause such a symptom, and no codes were ever set (ignition setup on these engines really doesn't get much benefit from diagnostics) so I had no reason to suspect it. If you have access to a spare ICM or can get one cheap from a salvage yard, I'd check that as a likely suspect, crazy as it sounds. There is no way to test it so far as I know, so a swap is about the only way to prove it out. Link to comment Share on other sites More sharing options...
39Buick Posted March 27, 2017 Topic Author Share Posted March 27, 2017 Ronnie I took your advice and drove the reatta to a safe spot , depressed the brake pedal and pushed the throttle to the floor. Stalled out about 2200 rpm. The bucking always occurs around 4000 rpm. As I drove home I accelerated in Drive, throttle to the floor and the bucking commenced around 4500 rpm. When I got home, I put the car in Park, and accelerated the engine to 5000 rpm without a burble. I still am thinking its in the tranny Link to comment Share on other sites More sharing options...
Ronnie Posted March 27, 2017 Share Posted March 27, 2017 You are in new territory for me. I never run my 3800 that hard. If you were able to go WFO and stall the converter at 2200 without problems it would seem the engine is in pretty good condition. You could possibly have a weak or broken valve spring causing the problem at high RPM. I've never experienced a transmission problem with bucking like you describe at high RPM but that might be it. Maybe our resident 3800 wizard, 2seater, will have some ideas. He has a turbocharged Reatta and he knows a lot about getting performance out of them. Link to comment Share on other sites More sharing options...
2seater Posted March 27, 2017 Share Posted March 27, 2017 I can't help but think it is an ignition break down issue. That type of issue shows up under load. Have you run it through a couple of gears manually to see if the behavior changes? You could close up the spark plug gap to .045" or so to help the ignition if it is indeed a mis-fire. Was the throttle valve cable reset after the manifold r&r? That's the other thing I can think of. If the cable is maladjusted, it may be hung in no man's land where it should have shifted. That is generally in the rpm range for a shift but it does vary with what gear it's in. It is hard to really rule anything out completely, as it is also getting into the mileage where a timing chain may perk things up too. If it has lived an easy life the chain takes less abuse and of course the reverse is also true. Hard to really Link to comment Share on other sites More sharing options...
DAVES89 Posted March 27, 2017 Share Posted March 27, 2017 I'm with kdirk on this except I will add the MAF sensor. Just because you recently replaced them doesn't mean they didn't go bad again. 5000 RPMs is a lot for this engine you might ruin a cam magnet that way... Link to comment Share on other sites More sharing options...
kdirk Posted March 28, 2017 Share Posted March 28, 2017 ICM is easy enough to swap, and certainly easier than dropping a transmission. I'd start there just on that basis. I have to reiterate that when I had this problem I really thought it was the transmission, it really felt like it was hunting gears rapidly. Never would've believed the ICM caused it, and resolved it by chance by having to replace the ICM due to complete failure. Dave is right to cite the MAF as a possible could be, but that's my assessment. Start with the easiest and cheapest stuff, then work your way up to the serious stuff. Link to comment Share on other sites More sharing options...
Frogware Posted March 28, 2017 Share Posted March 28, 2017 Hi Jim, I actually know what you are referring to, because mine was doing the same thing. It ran great under normal conditions but as soon as I floored it, just before it would shift into another gear it would act like a flopping fish jerking or bucking until I let off the gas and it would shift. I have just replaced my whole ICM but haven't had a chance to see if that fixed it. It has been raining on and off here the last couple weeks, but at soon as I get good weather I will try it and let you know my results. Link to comment Share on other sites More sharing options...
Frogware Posted March 30, 2017 Share Posted March 30, 2017 So it finally cleared up today and I decided to take the Girl for a quick spin. After it warmed up I stepped on it and she quickly responded. There was no hesitation or any more bucking, it even shifted into 2nd smoothly and responsively. I was surprised and happy, I tried it a few more times at different speeds and no more issues. If you have access to a spare ICM I would try that and see if that fixes it. Good Luck! Scott Link to comment Share on other sites More sharing options...
kdirk Posted March 30, 2017 Share Posted March 30, 2017 I,do have one follow up question. Are you still running the stock Magnavox ignition (with all 3 coil packs as one integral unit) or has yours been upgraded to the Delco ignitio setup with the three separately serviceable coil packs? I ask only because my experience with this failure mode was running a Delco setup (stock for 1991, the Magnavox setup was factory installed on all 88-90 cars) and I'm curious if the Magnavox ICM is capable of exhibiting the very same failure. In any case, I'm pleased to hear it was fixed by swapping the ICM. You went down the same line of thought I did when I had this happen, and clearly arrived at the same impression of the symptom being a transmission problem. I'm thinking this problem is more common than most of us have considered. Since an ICM swap is the only way to diagnose it - on board diagnostics gives you no help at all in this regard - I wonder how many people have fruitlessly chased what they thought were transmission problems on a 3800 equipped car since the late 80's when this version of the engine was first offered. Link to comment Share on other sites More sharing options...
Frogware Posted March 30, 2017 Share Posted March 30, 2017 I,do have one follow up question. Are you still running the stock Magnavox ignition (with all 3 coil packs as one integral unit) or has yours been upgraded to the Delco ignitio setup with the three separately serviceable coil packs? I ask only because my experience with this failure mode was running a Delco setup (stock for 1991, the Magnavox setup was factory installed on all 88-90 cars) and I'm curious if the Magnavox ICM is capable of exhibiting the very same failure. In any case, I'm pleased to hear it was fixed by swapping the ICM. You went down the same line of thought I did when I had this happen, and clearly arrived at the same impression of the symptom being a transmission problem. I'm thinking this problem is more common than most of us have considered. Since an ICM swap is the only way to diagnose it - on board diagnostics gives you no help at all in this regard - I wonder how many people have fruitlessly chased what they thought were transmission problems on a 3800 equipped car since the late 80's when this version of the engine was first offered. Ooops! I did forget to mention that I upgraded mine to the new Delco setup. kdirk you do pose a very interesting question whether this is misdiagnosed more often than not. As for Jim with the original issue I wonder if he replaced his ICM with the stock setup, if so that would explain his possible same symptoms. I think I was reading somewhere (Maybe the Tutorial) about the Magnavox ICM that GM knew/know the issues of the setup and agree that it was good at the time and worked but not for the long term. So they expected the ICM would eventually fail. Also the Delco setup is much more responsive and has a better and faster spark, among many other improved specs. It has been mentioned by many on the forum that it would be very beneficial to upgrade your ICM to the Delco setup. Link to comment Share on other sites More sharing options...
Ronnie Posted March 30, 2017 Share Posted March 30, 2017 So it finally cleared up today and I decided to take the Girl for a quick spin. After it warmed up I stepped on it and she quickly responded. There was no hesitation or any more bucking, it even shifted into 2nd smoothly and responsively. I was surprised and happy, I tried it a few more times at different speeds and no more issues. If you have access to a spare ICM I would try that and see if that fixes it. Good Luck! Scott When you replaced the ICM did you replace the coils as well? Link to comment Share on other sites More sharing options...
Frogware Posted March 31, 2017 Share Posted March 31, 2017 When you replaced the ICM did you replace the coils as well? Ronnie, Yes I did, I replaced all of it - The plate and coils. The next thing on the list is changing the plugs and the wires, I think they are still factory with over 178,000. Link to comment Share on other sites More sharing options...
39Buick Posted April 2, 2017 Topic Author Share Posted April 2, 2017 I,do have one follow up question. Are you still running the stock Magnavox ignition (with all 3 coil packs as one integral unit) or has yours been upgraded to the Delco ignitio setup with the three separately serviceable coil packs? I ask only because my experience with this failure mode was running a Delco setup (stock for 1991, the Magnavox setup was factory installed on all 88-90 cars) and I'm curious if the Magnavox ICM is capable of exhibiting the very same failure. In any case, I'm pleased to hear it was fixed by swapping the ICM. You went down the same line of thought I did when I had this happen, and clearly arrived at the same impression of the symptom being a transmission problem. I'm thinking this problem is more common than most of us have considered. Since an ICM swap is the only way to diagnose it - on board diagnostics gives you no help at all in this regard - I wonder how many people have fruitlessly chased what they thought were transmission problems on a 3800 equipped car since the late 80's when this version of the engine was first offered. The ignition coil is similar to the stock Magnavox ignition-all 3 coil packs as one integral unit. I have seen the post by frogware and he had a similar problem with his reatta and it was cured by replacing the ICM. My ICM is only a little more than a year old and I will replace it. I will be out of town till the 15th, so I'll get to it when I get back. Thanks to all for the advice, ideas, and input Link to comment Share on other sites More sharing options...
39Buick Posted April 20, 2017 Topic Author Share Posted April 20, 2017 I was out of the country for a couple of weeks and got home about a week ago. I took the advice of many of you who responded to my query on the bucking problem when engaging passing gear. Frogware was instrumental in my actions because he had exactly the same problem as I did. Yesterday I replaced the ICM and the coil pack ( event though it was installed about 15 months ago) and my test ride was exhilarating -no bucking and a powerful smooth surge when engaging the passing gear. Problem solved--I just had to convince myself that there was a chance that the old ICM was faulty from the first day it was installed. Thanks to all that responded to my query Link to comment Share on other sites More sharing options...
Ronnie Posted April 20, 2017 Share Posted April 20, 2017 Thanks for letting us know what fixed it. We all learn from it when we find out what corrects a problem. Link to comment Share on other sites More sharing options...
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