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Swapping HT4100 (4.1L V8) Fleetwood Brougham with a 5.7L V8

4057 Views 5 Replies 4 Participants Last post by  kennethfjoe
I have a 1985 Fleetwood Brougham RWD with a 4.1L and a THM200-4R transmission. However, it started leaking engine coolant. After inspection, it is somewhere from the back for the engine - very possibly engine block. I am looking to replace this engine. I found a 1990 Cadillac Brougham with a 5.7L engine. The 1990 model has an electronic speedometer, etc., and as far as I researched a THM700-4R transmission. Is it possible to swap the 5.7L engine into my 1985? Will it work with my current transmission, or will I have to include a transmission swap? I really appreciate advice.
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UPDATE: on my 1985 Fleetwood Brougham with an HT4100. It developed a coolant leak, which increased over time. It was somewhere behind the engine, not really visible. I took it to a repair shop to do a pressure test, and the mechanic pointed out that it was a coolant temp sensor junction block leak, but not very easy to see. The diagnostic cost me $94.95.

Initially, when we just got the car, I didn't pay attention to add the GM Cooling System Seal Tablets, (which Jim mentioned his response above) after flushing the cooling system. Yet, it does say to add those things under the hood. After researching on forums and looking around, I bought these tablets on Amazon:

Bought 2 packs - 5 tablets in each pack. I believe we used 5 tabs initially (crushed tabs in a plastic bag first and then mixed that powder with coolant and poured into radiator). After adding them we let the car run for a while and drove around and I rejoiced that the leak stopped. But, my joy was short-lived! Next morning, I come in and check under the car - there's a wet spot from the dripping and I could see drops of coolant coming down. The tablets didn't work for me.

It came to the point where it wasn't just dripping, it was almost flowing. At one point I noticed three areas it was leaking from under the car. I started using just distilled water instead of green antifreeze, since it was cheaper. After some days/weeks I added 2 or 3 more remaining tablets – it did not help. Coolant was still leaking. One thing I want to mention is that coolant was only leaking when the car was not running. It did not leak when the car was running. I was at a point where I just wanted to sell the car, because the only real fix that I researched (especially with what’s said about HT4100s) would be to replace the engine. I already started to look for a 5.0L or a 5.7L Cadillac Broughams. In essence, I was looking for a Brougham from 1986 -1992, which had almost identical body style and good engines. The thinking is that it would reduce work with fabrication of engine mounts and transmission fitting.

However, last week I was just Googling for information related to coolant leaking and discovered a video about K&W Fiberlock Head Gasket & Block Repair. Here it is: (
)I shared it with my dad and we decided to give it a try. I ordered it on Amazon:
As you can see, it’s less than $30 as it is today, on August 31,s 2019. We did all we could by this point, and thought that - let’s try it. If we are thinking of replacing the engine anyway….let’s give it a try.

I got this product this past Monday. And you know before going to put this product into the radiator, I kneeled and prayed in my room. I said - I don’t know how this remedy works, or what’s wrong with the coolant leaking out, but I asked God to help me with this. The car was cold. Dad filled the radiator and coolant reservoir with distilled water, even though water was leaking out from underneath the car. We started the car. My dad and my brother and I used about a quarter to a third of the K&W bottle. I poured it into the radiator and closed the radiator cap. We let the car run for about 20 minutes, then drove it/let it run for, I think, 40 or so minutes.

I come to the car this past Tuesday morning, look underneath – there’s just a very slight wet spot from coolant and I think I only saw one or two drops come down. I looked in the coolant reservoir – coolant was still in there! I was so glad, because previously, my coolant reservoir would empty overnight! I took the car to work this past Thursday – and I did not have to add distilled water. I monitored the coolant level and it remained steady, appearing to suck some out of the reservoir as it cools down, but then filling it back up. There was maybe one small wet stop I noticed on Thursday in the morning. I took the car to work again, yesterday, on Friday – and praise God – there is no coolant leaking anymore!!! I am still monitoring it, but it’s so refreshing to know that we don’t have to replace the engine.

I really enjoy driving this car – it’s a really good experience – it floats on the road. It’s a different era. And if this solution would help anyone with their car’s problem – its glory to God and thank to the manufactures of K&W Fiberlock Head Gasket and Block Repair. I just shared what I went through. Note, if you decide to use this product, make sure to read the directions on the bottle thoroughly. Do this when the car is not warmed up, there’s specific things you have to do (i.e., opening radiator cap, starting the car, turning on heat on max, etc.) when adding this product to your cooling system. Read the directions thoroughly and watch the video – it should clarify. If you have any questions – please ask – I’ll be glad to share what we have went through. Good luck with your car and God bless!
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UPDATE: Coolant is still leaking, unfortunately. The K&W works, but in this case with a 4.1L engine, we think because two different metals are used for the engine - aluminum and iron (correct me if I'm wrong) - they expand differently and whatever gap gets sealed with this solution - it gets pulled apart or torn apart as they warmup/cool down. Unfortunate.
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