Engine 90 LX hesitation under load

HarshFyasko66 · Nov 30, 2018

Hi guys, been trying to hunt down this issue for awhile now but can’t seem to get it straight.
- Bought car from a guy who claims the previous owner has done work on the engine but isn’t sure what.
- when ever I get on the throttle remotely hard it will hesitate or miss erratically.
- I have gone through the entire idle surge checklist with no luck of a solution.
- idle seems fine cold or warm about 650-700 rpm
- checked time set to 10 degrees, I have tried to bump up and down but no change.
- no CEL KOEO shows code 11
- ran a compression test all cylinders are within 10psi of each other
- checked each sensor from EGR to MAF but no change.
- I ran pin tests on the ECM no shorts or grounding issues, but I did notice if I tapped the side of it or wigged the wires while running the engine would stumble a bit.
- fuel pressure was fine also just replaced pump and filter.

I am completely out of ideas and just want my girl to run. Open to any suggestions.

Wanted to post a video but they are too large.

Thanks in advance for any help.

HarshFyasko66 · Nov 30, 2018

Oh and I get a lot of backfire when off the throttle.

General karthief · Nov 30, 2018

HarshFyasko66 said:
but I did notice if I tapped the side of it or wigged the wires while running the engine would stumble a bit.

there is a hint on what the problem is, clean and make sure the pins are tight.

HarshFyasko66 · Dec 1, 2018

Thanks I will pull it out again and really dig into it

jrichker · Dec 1, 2018

High speed miss on a warm engine

Revised 7 Nov-2016 to add PIP sensor as possible problem and dumping the codes to help determine if it is the TFI or PIP.

The TFI module mounted on the distributor is one of the culprits for a high speed miss on a warm engine. The other suspect is the PIP sensor inside the distributor. If the problem does not occur when the engine is cold, the TFI module or PIP is definitely suspect. Dumping the codes may help determine which one it is. You may need a special socket to remove the TFI module, but most auto parts stores will have one for $5-$7.

Be sure to use plenty of the heat sink grease on the new TFI and clean the old grease off the distributor.

See Automotive Tools Specialty | Auto Mechanic & Technician Diagnostic, Testing Equipment | Thexton

diagram courtesy of Tmoss & Stang&2Birds

HarshFyasko66 · Dec 2, 2018

Thought the same and have swapped the TFI with a new one and no change. As for the PIP I have gotten to it yet. I will take a look after I check the pins on the ECM

Steel1 · Dec 2, 2018

Does it still have the stock dizzy, specifically the shutter wheel ?

HarshFyasko66 · Dec 10, 2018

I’m not sure how to determine a stock one but I can say that it looks old. I wanted to try to replace with a new one but I don’t know what cam is in the engine aka cast or steel. And I didn’t want to risk it.

HarshFyasko66 · Dec 11, 2018

So I finally had time to work on the car. I removed the ECM and opened up all the pins and thoroughly cleaned them. Then did the same for the female end.
Started her up and everything seems to run great. Took her for a quick drive after she warmed up. All of my throttle response is back and no more hesitation. Will go on a longer drive tomorrow to see how she fares. Will update when i get back .

Oh, and thank you to everyone for the help and tips!!!

a91what · Dec 11, 2018

Its amazing what a bad electrical connection will do to a EFI engine. Glad its running good for you again. got any pictures of it?

HarshFyasko66 · Dec 14, 2018

Spoke a bit too soon. Same issues but now it only happens sometimes. It doesn’t seem to show any patterns of the issues either so I’m at a complete loss on where to go next. Gonna run a smoke vacuum test on the intake side and exhaust side to see what happens.

HarshFyasko66 · Dec 14, 2018

a91what said:
Its amazing what a bad electrical connection will do to a EFI engine. Glad its running good for you again. got any pictures of it?

HarshFyasko66 · Dec 14, 2018

So I was in neutral coasting to a stop. Car started to surge from 600-1000 rpm then died. She started right back up with no issues. The surge doesn’t happen at idle.

General karthief · Dec 14, 2018

Go through the surging idle checklist, you'll find it on the 5.0 tech forum under 5.0 technical how to thread, start at the top of the list, do it step by step, don't skip around.

HarshFyasko66 · Dec 17, 2018

I have gone through the checklist before with no avail. I will definitely try again maybe I missed something. Since I’m pretty sure the previous owner has done some work I decided to measure the throttle body and came out with 63-64 mm in measurement. I know the stock is 56mm. I replaced the MAF sensors with an OEM replacement and turned in the old one for core charge. Would this new MAF have issues with a larger throttle body?

General karthief · Dec 17, 2018

Nope. As long as it matches the injector size. Go back through the checklist, don't skip around. I took me a couple times, I read through it to get an understanding and made notes as I went.

HarshFyasko66 · Dec 19, 2018

vaccum check found a small leak from the oil fill cap got it fixed but not much change. Running really rich can smell gas and noticed more fuel consumption on commute to work.

Forgot to mention that I’m currently only running headers do to the old exhaust system doesn’t mate up with the new long tubes. Will that effect much?

General karthief · Dec 19, 2018

Yes, hearing loss!

HarshFyasko66 · Dec 20, 2018

Yeah I figured that out pretty quick. I keep a set of earpro in the car now.

Also I drove the car to work today which is about a 30 min drive. Seemed to have random misfires under acceleration. No pattern or consistency with it at all.

General karthief · Dec 20, 2018

Lack of exhaust back pressure will make a street engine run funny

Engine 90 LX hesitation under load

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