I have an 86 with a 5.0l Sefi engine with speed density system. At part throttle cruise and part throttle acceleration at any speed the engine will miss. I misses noticable so that the vehicle will surge.

At full throttle the thing runs fine and seems to have a good amount of power. It idles fine.

There is no check engine light on.

I ran the codes and I get lean readings on both oxgen sensors as codes. I was getting a TPS code also. I replaced that and ajusted it and got no impovement, but the code went away. I have not disconnected the battery at all. That's it, just continues memory codes indicating lean HEGO codes.

I ran a cylinder balance test and the engine passes with flying colors.

The vehicle was sitting for about a year and a half and had this problem as soon as it left storage. Not sure if it was there before as I didn't own it. I though maybe bad gas but it is quite a few tanks later and I ran a few cans of seafoam injector cleaner through it to. I put a new fuel filter on it. Heat elements in the HEGO sensors test fine. Not sure what the point of testing the output of the HEGO would be cause good or bad they should just read lean like the computer says they are.

Gas milage sucks with engine running like this. 11-12 mpg. I think it should be 16 plus mpg. Engine has about 130,000 miles but seems to be in good condition otherwise.

I searched the forums but had trouble finding anything good about this problem.

Thanks for any help you can give.

Matt

 

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clean the IAC. might be time for a tune up as well. whats your timing set at and you might think about replaceing the 1 o2 sensor

 

I like to know the why's not just the what to do's. I have no problem following your suggestions but why.

Okay first IAC. At part throttle shouldn't it be 100% open? I am referencing information found on this website

http://fordfuelinjection.com/?p=64

Under the warm cruise section.

From my understanding the Iac is suposed to be at 100% in preparation for braking and deceleration throughout the driving cycle minus deceleration and idle.

The car does not stall or run poorly at idles so I guess I assume it does work like it is suposed to. It doesn't hurt to clean it but what would it be doing to cause my problem.

My timing is set at 10 degrees done following factory instructions.

I have changed all the spark plugs in attempts to fix problem. The old ones were light-tan and loaded with deposits. But none exibited signs of extended lean condition. The new ones improved idle and WOT but not the miss problem. I inspected the plug wires and found nothing wrong with them and saw no flash or arch in a pitch dark shop. I changed the cap and rotor.

I have 2 O2 sensors that are heated(I have a confession, it's a 1986 Mercury fullsize we are discusing. though I do own half a dozen Ford trucks they aren't giving me fits at the moment.

Would bad o2 sensors really cause the miss? If you think so then I will replace them. Is there a way to test them that will really tell me if they are bad?

For kicks I will clean the IAC and let you know if that helps. Got any other suggestions for a tune-up or otherwise?

Thanks,

Matt

 

I cleaned the iac. No change in performance. It still misses at part throttle acceleration. Any more ideas?

Matt

 

have you tested the TPS voltage? if the o2 sensors were messed up they would throw a code. that web site should explain how to get the codes using a paperclip. strange that you have 2 o2 sensors though there should only be 1. how old are the plug wires and what type of spark plug are you running?

 

I tested the tps voltage and ajusted it after installing the new one, per specs on the fordfuelinjection site.

I drove the thing for about 500 miles. This weekend I took it to autozone and had them scan the codes. I know how to do it but am a bit lazy.

I reset the memory cause I ended up getting a bunch of codes from me trying different things(conecting and disconecting things to see what happened) and the tps code was still present. So I reset the codes on sunday and will check them sometime again over the holiday.

I'm not actually talking about a truck. I am working on a 1986 Mercury Grand Marquis. My ford trucks run fine. So that is where the 2 o2 sensors come in.

The plug wires don't look real old but I have know idea how old. They don't have any damage to them. I have heard bad wires will throw sparks and maybe glow in the dark. So I checked them in a very dark garage and didn't see anything.

I am running autolite resistor plugs. They are the standard range for the car.

I am trying to not throw money at the problem. I guess that is why I didn't do anything with the wires. I'm going to get a fuel pressure guage and check that. Not sure why that might cause a problem, but searching some of the other posts in the forum, it seems like standard practice when looking for problems.

Thanks for your help,

Matt

 

might even try a fuel filter replacement. if you do get o2 sensors get them from the dealer. the parts are better

 

i have replaced the fuel filter. no difference.