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Hello everyone! I am very excited to see that there is a forum for these cars. I recently picked up an 88 Plymouth Horizon for the price of free. It had sat for several years and it came so close to the junkyard. Replaced broken fuel line, fuel injector, flushed all
fluids and she fired up. It would not idle and after replacing the IAC valve and replacing most vacuum lines it was found that the
MAP sensor was at fault. Now it runs like a champ. Got her legal and it is now my DD to keep the miles of my GT500. One problem I am having though is this awful knock. I originally thought it was mechanical so I put some 20W 50 in there to try and
quiet it down. After doing this with no success I have determined that it must be a very bad spark knock. Reason being is that when you first start the engine when cold it will idle perfect with no knock for about a minute and then it will set in. It seems that one of the sensors may be warming up and feeding the computer a bad signal causing a timing advance issue or maybe a bad fuel mixture. I ordered a new knock sensor but after 2 hours spent looking at every nook and cranny of that engine I believe that there is none. The next item I have ordered is a new distributor cap. The plugs and wires are new. The car has no check engine light but then again it did not when the MAP sensor was bad either. I really need new tires and want to put some fresh paint on there and it should look as good as new. I am however, reluctant to do so with the likely engine failure associated with this knock. Also the previous owner saw this video that I am going to post a link to and while laughing identified this as his old car from the distinct engine knock. This car has knocked since he got it in 2002 and up until it was parked in 2009. In the video you will clearly hear it. Thanks in advance for any advice, knowledge, or help!!!

http://www.youtube.com/watch?v=dwMZ7iSNYdI
 

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Welcome to Allpar. Being EFI, it does have a 'ck eng' or 'power loss' warning light. The bulb may be out or some owners may have pulled the bulb instead of fixing the problem. Its location should be shown in your owners or service manual.
The first job is diagnosis and locating the noise source. Is it upper or lower end? I doubt that it is fuel knock if it occurs at idle and closed throttle. Carbon knock is possible and some fuel additives like Techron may help.
It possibly has the roller cam that ran quieter than the conventional rockers. Wrist pin noise was also common, but these engines kept running for years without coming apart.

Factory service manuals this may help keep the car in good shape and be a priceless information tool:

http://www.faxonautoliterature.com/1988-MoPar-FWD-Car-Repair-Shop-Manual-3-Vol-Set-P16299.aspx

http://www.ebay.com/itm/1988-PLYMOUTH-HORIZON-FACTORY-ORIGINAL-OWNERS-MANUAL-/200520923074
 

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There are no lights going to indicate a problem. The car actually runs great. As far as a bulb missing I cannot verify just yet but will check. I understand that this noise would likely be from a mechanical failure but as stated before it absolutely does not knock at all and idles perfect when the engine and sensors are cold. Not temperature cold but not warmed up to operating temperatures cold for lack of a better way to describe. If you remove the oil cap on the valve cover while the engine is running it is very clear that the noise is not coming from the top of the head but rather the cylinder valve side or below. If this were a true mechanical failure it would have blown up many years ago. It must be a detonation knock. Whether it is caused by timing, high compression due to carbon, or air to fuel mixture I cannot determine. I am leaning toward the electronic sensor route cause they are very cheap and easy to replace. I did run a tank of 93 octane with a fuel injector tb cleaner and it had zero effect. I am now running 87 without additive and it is the same. Can anybody verify whether or not these cars have a knock sensor? They sure do sell them. I went through all 58 pages of Omni Horizon posts last night and could not find any similar problems. I did read a post that stated that if the thermostat or coolant sensor were to be inaccurate (but still functional) that your computer would think that the car is still cold and cause it to run rich. I don't think I have a problem with this but it may be worth looking into. The car never does warm up to midscale but rather stays on the lower end. This may be due to the fact that I am only running distilled water at the moment (due to a leak I have since fixed). Again any help is appreciated as I am by no means a mechanic. I have thought about taking to a mechanic cause it is such a great little car and could last quite some time still.
 

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I believe that only the turbo versions had knock sensors.
A mechanic (or two's) opinion on the noise may help give some direction on what to pursue next.
If the car has a working 'ck eng' light, it should come on briefly at key-on for a bulb light check.
Straight water isn't good to keep that way, it will boil at a much lower temperature than the proper 50/50 mixture and doesn't provide lubrication for the water pump seal.
These cars had limited self-diagnostics like counting the 'ck eng' light flashes for fault codes and/or DRB II/III scan tool diagnostics.

http://www.allpar.com/fix/codes/index.html
 

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Thank you. I will check the power loss light. I know the CEL works but do not recall seeing the other. That would be great if the bulb is burnt out as it may lead to some direction as to what may be failing. If that does not lead to anything then a trip to a mechanic may be in order.
 

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There is no knock sensor on the TBI cars.

Have you checked the ignition timing to verify that it's correct? It should be 12 degrees BTDC, with the Coolant Temperature Sensor unplugged.
 

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Wrist pin knock was prevalent on some of the 4 cylinder engines from that era. It kind of made the engine sound like an old diesel engine. Other than the annoying noise, there were no other issues. I had an 88 2.5 engine that made this noise for 70,000 to 80,000 miles and it always ran just fine. I don't have a recording of the noise but it was kind of like a clack clack clack ....noise. :)
 

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And often, synthetic oil would make that noise go away.
 

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If it were a wrist pin knock wouldn't this occur as soon as the engine is started from cold as opposed to setting in after a minute or so? The idle speed changes a bit as soon as it sets in too. I feel that I do need to check the timing. How do I check the timing? I do need to replace the oil pan gasket so I will give the synthetic oil a try. As for the fault codes I have not checked yet. Car has been at work but I will try this as soon as I get to bring it home. I also ordered a new distributor cap as well to see if this helps. If it does not then oh well, it was only $5.00 and probably needed replaced soon anyways. I am going to shoot a video of the car starting up from cold and post it on here to see what you all think. Its just very weird that it idles perfectly for a short period of time and like clockwork that knock will set in. Thanks everyone.
 

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It actually sounds like a lifter sticking more than anything, although it could be a wristpin. My reasoning is there didn't seem to be a time when it didn't knock at idle, wristpins in my experience can have misses where it doesn't make a sound, and increase with rpm, then quiet when backing off.
 
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