Help! 99 Gt Intermittently Misfiring

bohan

New Member
Dec 6, 2016
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i have a 99 GT auto vert with about 125k miles. i bought the car in december of last year for dirt cheap (thing was leaking coolant bad, among other things). since then it's pretty much been a continuous project as it is my daily driver. I've been able to tackle most issues with it, but now i'm stuck.

The engine began running really rough recently. I parked it and went to work for about five hours and when i got back in the car it was driving like :poo:. I took to the forums, got the engine light read, and went to work throwing parts at it. the engine runs alright once RPMs are steady. however, accelerating is terrible. the engine stutters and just feels like it isn't getting any power. if i give it some gas, it'll run real bogged down like this for a bit and i can feel it kick and go back to normal at maybe 3k RPMs? like it decides to run normally for the rest of that gear. My gas mileage is also :poo:e.

My engine began to steam one night i discovered the coolant hose on the left side of the engine had come apart in the middle, the connected had dry rotted and the tube was dumping coolant into the compartment. i fixed this issue by just connecting the one half of the coolant hose to where it lead to instead of a second tube. just figured this was worth mentioning.

also a couple months ago i had 4.10 gears installed, my speedo is off by 10 mph. i recently replaced two o2 sensors, the fuel filter, and the evap purge valve. the engine only misbehaves really under load, with a small dip every once in a while in park. i put the car on blocks and put it in drive and unplugged each coil connector one by one and every single one had an affect on the engine, even tho the engine was running like hell with all the coils in.

the misfire throws no code because it happens intermittently, and the engine light will flash if i accelerate too much. the car has no issue reaching high speeds, i can easily hold it going 65 without issue.

i don't know what else to do. anything helps at this point. i just want the sport back in my sportscar.
 
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90sickfox

I didn't really have an issue with the stink...
SN Certified Technician
Mar 2, 2015
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Sometimes its really worth it to have a mechanic diagnose it. It could be anything from compression to a temp sensor. There are way too many variables with an intermittent misfire.

Hate to tell you....but....sometimes its worth forking out the couple hundred bucks.
 

90sickfox

I didn't really have an issue with the stink...
SN Certified Technician
Mar 2, 2015
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Let me clarify... If you don't have the skill to diagnose it yourself ( with instruction ) then you need to pay someone that knows what they're doing.

First, inspect all vacuum lines.....get a vacuum gauge and check for vacuum at idle. Take a pic of the gauge when the cars running and post here.

Then, run a compression test. Some parts store will loan compression testers. Write the readings down for each cylinder and post here.

That's also a great time to inspect the spark plugs. The color of them is important. Number the plugs, take a pic and post it here.

I'm sure folks will chime in to help.

You'll also have to do an ohms test on each of your coils. Sometimes a weak coil won't set the CEL.

A set of noid lights would be great for checking the fuel injectors for correct pulse.

Harbour Freight is a great store to find these tools super cheap.
 
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soupnutz

Member
May 23, 2009
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Hampton Roads, VA
It idles fine and runs fine in the driveway for the most part and the issue is mostly when you're accelerating? No problems holding the speed once you get there? Does how much you push the gas pedal effect it? Can you ease the throttle with no problems and the issue is only when you give it, say, more the 1/4 throttle? There are a few sensors that could cause similar issues, but to me it sounds like a fuel delivery problem.
Have you changed the fuel filter since you've had it? That would be an easy first step. I don't have much exp with 99+'s but I believe they don't have a schrader valve to hook up a fuel pressure sensor like the npi's do. Maybe somebody else can chime in. If it's anything like my 05 F150, the fuel signal is electronic. I can hook my up SCT and it tells me what the pressure is on the datalogging screen. Another option is a bluetooth obd adapter and the torque app. For about $20 it works pretty well. I actually prefer using that over the SCT for casual datalogging and it works on my mustang, BMW, and F150.
Do you know how long this car sat before you got it? Not saying the problem is the same, but my car had very similar symptoms to yours after sitting for 2 1/2 years. Verified my fuel pressure was low and dropped the tank to find that there was 3/8 of an inch of crud built up on the bottom of the tank that was causing the pump to run dry.